There is no real to move to a 6500 except all models support redundancy whereas only the 4507R supports redundancy. So if you like running IOS only then there is nothing wrong with the 4500. Thet both support the same features except if mem serves me correct you can not use all 4 links on the 4500 SUPIV for redundancy at once only two at a time.
Also the supervisor slots on the 4500 can not accept line cards as with the 6500 slot2 can..
Hope this helps