Hello,
You can go for 4506 or 4507R.Both the chassis support 5 payload slots.The difference between them is the 4507R supports Sup Engine redundancy and provides you sub-mili second failover between the sup's. However there is no scalability left in the above solution as the chassis will be full.
My Suggestion for you would be look for Cat6509 with Sup32 engine. This will support more payload slots, more scalability easy migration to 10GE in future.
HTH,Please rate if it does.
-amit singh