We are planning to upgrade a pair of 5509's for a pair of 4703 with 2 x 48 port gig Ether cards(yes smaller but the 5509 was over capacity module wise) What's the best SUP card to use in the 4703 ? There's nothing fancy on the 5509's they are just used for routing to the Backup servers, so theres a lot of l3 interfaces and lots of traffic at night.
I think you are talking about 4507r chassis. Do you need to run the ehanced routing protocol like EIGRP, OSPF or BGP. If yes, then you would need Sup4 or Sup5, other wise Sup2 + will be a good choice.
If you need 10GB connectivity you can look for L3 Sup5-10GE or Layer2 Sup2+-10GE. In my opinion Sup2+ should suffice your need unless you want to run enhanced routing protocol.
Please have a look at the Sup engines for 4500 Chassis.
HTH,Please rate if it does.