page:
<apex:page standardcontroller="MPI_Category__c"  extensions="MPIController1" action="{!autoRun}"
  tabStyle="Contact">
  
   <apex:form >
   <apex:actionFunction name="rerenderSg1" rerender="sg1sSelectList1"  >
          <apex:param name="firstParam" assignTo="{!catalog}" value="" />
        
      </apex:actionFunction>
      <apex:actionFunction name="rerenderSg2" rerender="sg2sSelectList1"  >
          <apex:param name="firstParam" assignTo="{!sg1}" value="" />
        
      </apex:actionFunction>
      <apex:actionFunction name="rerenderSg3" rerender="sg3sSelectList1"  >
          <apex:param name="firstParam" assignTo="{!sg2}" value="" />
        
      </apex:actionFunction>
       <apex:actionFunction name="rerenderSg4" rerender="sg4sSelectList1"  >
          <apex:param name="firstParam" assignTo="{!sg3}" value="" />
        
      </apex:actionFunction>
     
      <apex:sectionHeader title="MPI {!mpi.name}"  subtitle="MPI Item Selection" />
<apex:pageBlock >
   <table><tbody>
      <tr>
        <th>Catalog</th>
          <td>
             <apex:selectList id="catalog" styleclass="std" size="1"
                value="{!catalog}" onchange="rerenderSg1(this.value)" >
                    <apex:selectOptions value="{!catalogsSelectList}"/>
             </apex:selectList>
          </td>
          <th>Segment1</th>
          <td>
         
            <apex:selectList id="sg1sSelectList1" styleclass="std" size="1"
                 value="{!sg1}" onchange="rerenderSg2(this.value)" >
                <apex:selectOptions value="{!sg1sSelectList1}"/>  
            </apex:selectList>
          
          </td>
          <th>Segment2</th>
          <td>
         
            <apex:selectList id="sg2sSelectList1" styleclass="std" size="1"
                 value="{!sg2}" onchange="rerenderSg3(this.value)"  >
                <apex:selectOptions value="{!sg2sSelectList1}"/>  
            </apex:selectList>
          
          </td>
          <th>Segment3</th>
          <td>
         
            <apex:selectList id="sg3sSelectList1" styleclass="std" size="1"
                 value="{!sg3}" onchange="rerenderSg4(this.value)" >
                <apex:selectOptions value="{!sg3sSelectList1}"/>  
            </apex:selectList>
          
          </td>
          <th>Segment4</th>
          <td>
         
            <apex:selectList id="sg4sSelectList1" styleclass="std" size="1"
                 value="{!sg4}" >
                <apex:selectOptions value="{!sg4sSelectList1}"/>  
            </apex:selectList>
          
          </td>
    
     
      </tr>
     </tbody>
     </table>
      <apex:commandButton action="{!addRow}" value="AddRow"  rerender="pgblckID" />&nbsp;&nbsp;&nbsp;
    <apex:commandButton action="{!clear}"  value="Clear " id="theButton1"    />
       </apex:pageblock>
       
   </apex:form>
</apex:page>

controller class:
 public with sharing class MPIController1 {
public MPI_Category__c mpi {get;set;}
public MPI_Category__c mpiObject {get;set;}
public MPIProducts__c mpi1 {get;set;}
public Boolean sg4Bool {get;set;}
public Boolean sg3Bool {get;set;}
public Boolean sg2Bool {get;set;}
public Boolean sg1Bool {get;set;}
ApexPages.StandardController controller;
Public Boolean ShowpageBlockFlag{get;set;}
List<SelectOption> optionsca {get;set;}
List<SelectOption> options1 {get;set;}
List<SelectOption> options2 {get;set;}
List<SelectOption> options3 {get;set;}
List<SelectOption> options4 {get;set;}
    public MPIController1(ApexPages.StandardController controller) {
    this.controller=controller;
     mpi=[Select  m.Name from MPI_Category__c m where m.id= :ApexPages.currentPage().getParameters().get('id')];
     mpiObject = new MPI_Category__c ();
mpiObject = (MPI_Category__c )controller.getRecord();

    }
public PageReference autoRun() {

      
        return null;

    }

    // Variables to store country and state selected by user
  
         public String catalog {get; set;}  
        
         public String sg1 {get;set;}
          public String sg2 {get;set;}
           public String sg3 {get;set;}
            public String sg4 {get;set;}
   
      // Generates catalog dropdown from country settings
        public List<SelectOption> getCatalogsSelectList()
         {
           optionsca = new List<SelectOption>();
            optionsca.add(new SelectOption('', '--SelectOne--'));       

           // Find all the catalogs in the custom setting
          List<product2> prd=  [select id, name, MPI_Catalog__c,MPI_Sg1__c,MPI_Sg2__c,MPI_Sg3__c,MPI_Sg4__c from product2 where MPI_Catalog__c!=null];
         
          set<String> s= new Set<String>();
         
         
         
         
        for(Product2 pr:prd)
        {
         s.add(pr.MPI_Catalog__c);
         
        }
       
        List<String> list1= new List<String>();
       
       
        list1.addAll(s);
       list1.sort();
      
           
            for(String s1:list1)
            {
            optionsca.add(new SelectOption(s1, s1));
            }
        return optionsca;
    }
   
     public List<SelectOption> getSg1sSelectList1() {
     System.debug('entered1-----------------------');
      System.debug('entered1-catalog----------------------'+catalog);
      optionsca.clear();
        options1= new List<SelectOption>();
        // Find all the states we have in custom settings.
       
      
        List<product2> allsg1 =[select  MPI_Catalog__c,MPI_Sg1__c from product2 where MPI_Catalog__c=:catalog and MPI_Catalog__c!=null];
    if(allsg1.size()>0)
    {
   System.debug('entered11-----------------------');

        // Filter states that belong to the selected country
        Map<String, Product2> sg1s = new Map<String, Product2>();
        for(Product2 sg19 : allsg1) {
            if (sg19.MPI_Catalog__c == this.catalog) {
                sg1s.put(sg19.MPI_Sg1__c, sg19);
            }
        }
       
        // Sort the states based on their names
        List<String> sg1Names = new List<String>();
        sg1Names.addAll(sg1s.keySet());
        sg1Names.sort();
        System.debug('entered1---sg1Names -------'+sg1Names);
        // Generate the Select Options based on the final sorted list
        for (String sg1Name : sg1Names) {
            Product2 state = sg1s.get(sg1Name);
             System.debug('entered1---state  -------'+state);
            options1.add(new SelectOption(state.MPI_Sg1__c, state.MPI_Sg1__c));
        }
        }
       
        // If no states are found, just say not required in the dropdown.
        if (options1.size() > 0) {
      
            options1.add(0, new SelectOption('', '--Select One--'));
           
        } else {
        
            options1.add(new SelectOption('', 'Not Required'));
          
        }
        
        return options1;
    }
    public List<SelectOption> getSg2sSelectList1() {
    optionsca.clear();
    options1.clear();
    System.debug('entered2-----------------------');
        options2 = new List<SelectOption>();
        options2.clear();
       
        List<product2> allsg2 =[select  MPI_Catalog__c,MPI_Sg1__c,MPI_Sg2__c from product2 where MPI_Sg1__c=:sg1 and MPI_Sg1__c!=null and MPI_Sg2__c!=null and MPI_Catalog__c=:catalog ];
        System.debug('query is----'+[select  MPI_Catalog__c,MPI_Sg1__c,MPI_Sg2__c from product2 where MPI_Sg1__c=:sg1 and MPI_Sg1__c!=null and MPI_Sg2__c!=null and MPI_Catalog__c=:catalog] );
        if(allsg2.size()>0)
        {System.debug('entered21-----------------------'+allsg2);
        // Filter states that belong to the selected country
        Map<String, Product2> sg1s = new Map<String, Product2>();
        for(Product2 sg11 : allsg2) {
            if (sg11.MPI_Sg1__c == this.sg1) {
                sg1s.put(sg11.MPI_Sg2__c, sg11);
                System.debug('entered22-this.sg1----------------------'+this.sg1);
                System.debug('entered22-sg11.MPI_Sg1__c----------------------'+sg11.MPI_Sg1__c);
                System.debug('entered22-sg1s----------------------'+sg1s);
            }
        }
       
        // Sort the states based on their names
        List<String> sg1Names = new List<String>();
        sg1Names.clear();
        sg1Names.addAll(sg1s.keySet());
        sg1Names.sort();
      
       
         System.debug('entered23-sg1s----------------------'+sg1Names);
       
        // Generate the Select Options based on the final sorted list
        for (String sg1Name : sg1Names) {
            Product2 state = sg1s.get(sg1Name);
            options2.add(new SelectOption(state.MPI_Sg2__c, state.MPI_Sg2__c));
             System.debug('options2.add1---------'+options2);
        }
       
        }
       
        // If no states are found, just say not required in the dropdown.
        if (options2.size() > 0) {
       System.debug('options2.add2---------'+options2);
            options2.add(0, new SelectOption('', '--Select One--'));
           
        } else {
        System.debug('options2.add3---------'+options2);
            options2.add(new SelectOption('', 'Not Required'));
          
        }System.debug('options2.add4---------'+options2);
        return options2;
    }
    public List<SelectOption> getSg3sSelectList1() {
    System.debug('entered3-----------------------');
     optionsca.clear();
    options1.clear();
    options2.clear();
        options3 = new List<SelectOption>();
        options3.clear();
        List<product2> allsg2 =[select  MPI_Catalog__c,MPI_Sg1__c,MPI_Sg2__c,MPI_Sg3__c from product2 where MPI_Sg2__c=:sg2 and MPI_Sg2__c!=null and MPI_Sg3__c!=null and MPI_Catalog__c=:catalog and MPI_Sg1__c=:sg1 ];
        System.debug('sg2is-------------------'+sg2);
        System.debug('allsg2 -------------------'+allsg2);
        if(allsg2.size()>0)
        {
        // Filter states that belong to the selected country
        Map<String, Product2> sg1s = new Map<String, Product2>();
        for(Product2 sg12 : allsg2) {
            if (sg12.MPI_Sg2__c == this.sg2) {
                sg1s.put(sg12.MPI_Sg3__c, sg12);
            }
        }
       
        // Sort the states based on their names
        List<String> sg1Names = new List<String>();
        sg1Names.clear();
        sg1Names.addAll(sg1s.keySet());
        sg1Names.sort();
       
        // Generate the Select Options based on the final sorted list
        for (String sg1Name : sg1Names) {
            Product2 state = sg1s.get(sg1Name);
            options3.add(new SelectOption(state.MPI_Sg3__c, state.MPI_Sg3__c));
        }
       
        }
       
        // If no states are found, just say not required in the dropdown.
        if (options3.size() > 0) {
       
            options3.add(0, new SelectOption('', '--Select One--'));
           
        } else {
       
            options3.add(new SelectOption('', 'Not Required'));
          
        }
        return options3;
    }
    public List<SelectOption> getSg4sSelectList1() {
    System.debug('entered4-----------------------');
    optionsca.clear();
    options1.clear();
    options2.clear();
    options3.clear();
        options4 = new List<SelectOption>();
        options4.clear();
        List<product2> allsg2 =[select  MPI_Catalog__c,MPI_Sg1__c,MPI_Sg2__c,MPI_Sg3__c,MPI_Sg4__c from product2 where MPI_Sg3__c=:sg3 and MPI_Sg3__c!=null and MPI_Sg4__c!=null and MPI_Catalog__c=:catalog and MPI_Sg1__c=:sg1 and MPI_Sg2__c=:sg2];
        System.debug('sg2is-------------------'+sg2);
        System.debug('allsg2 -------------------'+allsg2);
        if(allsg2.size()>0)
        {
        // Filter states that belong to the selected country
        Map<String, Product2> sg1s = new Map<String, Product2>();
        for(Product2 sg13 : allsg2) {
            if (sg13.MPI_Sg3__c == this.sg3) {
                sg1s.put(sg13.MPI_Sg4__c, sg13);
            }
        }
       
        // Sort the states based on their names
        List<String> sg1Names = new List<String>();
        sg1Names.addAll(sg1s.keySet());
        sg1Names.sort();
       
        // Generate the Select Options based on the final sorted list
        for (String sg1Name : sg1Names) {
            Product2 state = sg1s.get(sg1Name);
            options4.add(new SelectOption(state.MPI_Sg4__c, state.MPI_Sg4__c));
        }
       
        }
       
        // If no states are found, just say not required in the dropdown.
        if (options4.size() > 0) {
       
            options4.add(0, new SelectOption('', '--Select One--'));
           
        } else {
       
            options4.add(new SelectOption('', 'Not Required'));
          
        }
        return options4;
    }
    public void addRow()
    {
    mpi1= new MPIProducts__c();
    System.debug('catalog is------'+catalog);
    System.debug('sg1 is------'+sg1);
    System.debug('sg2 is------'+sg2);
     System.debug('sg3 is------'+sg3);
     System.debug('sg3 is------'+sg4);
     if(catalog!=null)
     {
     mpi1.catlogp__c=catalog;
     mpi1.Name=catalog;
     }
     if(sg1!=null)
    {
    mpi1.sg1p__c=sg1;
    }
    if(sg2!=null)
    {
    mpi1.sg2p__c=sg2;
    }
    if(sg3!=null)
    {
    mpi1.sg3p__c=sg3;
    }
    if(sg4!=null)
    {
    mpi1.sg4p__c=sg4;
    }
   mpi1.MPIProductLo__c=(String)mpi.id;
  
   List<product2> itemdes=[select ProductCode,id from product2 where MPI_Catalog__c =:catalog and MPI_Sg1__c =:sg1 and MPI_Sg2__c =:sg2 and MPI_Sg3__c =:sg3 and MPI_Sg4__c =:sg4];

mpi1.ItemCode__c=itemdes[0].ProductCode;
  
   insert mpi1;
  
    ShowpageBlockFlag = true;
    }
    public pagereference clear()
    {
    return controller.cancel();
    }
    public pagereference update1()
    {
  
   
    update mpi1;
     PageReference reRend = new PageReference('/'+mpi.id);  
         return reRend;
    }
   
find the below screen shot for the same, whenever i will change  'catalog' values then that section should refresh and all sg1,sg2,sg3,sg4 as'non required'.
but it is showing with old values, 
thanks in advance

   
   
    }he sa