Modify the IRULE

This should work for you:

when HTTP_REQUEST { 
  Pool name 
 set thirdpool "pool3" 
 set forthpool "pool4" 
 set secondpool "pool2" 
 set firstpool "pool1"

switch -glob [string tolower [HTTP::uri]] {
    "/disk/disk.xml*"
    {
        reject
    }
    "/abcfirst*"
    {
        pool $firstpool
    }
    "/_site*" -
    "//_site*"
    {
        persist cookie insert "SessMgt"
        pool $secondpool
    }  
    "/us-home*" -
    "//us-home*"
    {
        persist cookie insert "SessMgt"
        pool $thirdpool 
    } 
    default {
        persist cookie insert "SessMgt"
        pool $forthpool 
    }
 }
}