Look at the BRANCH or BRANCHL commands.
Code:
Main:
   PortVal = PORTB & %00001111 ' Assumes lower 4-bits
   ' Or
   ' PortVal = PORTB >> 4 ' Assumes upper 4-bits
   BRANCH PortVal, [Nada, itsA, itsB, itsC, etc,,]
   GOTO MAIN

Nada:  ' PortVal returned 0
   GOTO Main

itsA:  ' PortVal returned 1
   do something
   GOTO Main

itsB: ' PortVal returned 2
   do something else
   GOTO Main

itsC: ' PortVal returned 3
   do something else
   GOTO Main
Much easier than 16 IF THENs.