Thank for your call regarding running BGP on Cat4500 SupIV.  Here is what I have gathered so far
On the Cat4500 SupIV, with 512M of SDRAM we have tested with up to 300,000 routes in BGP.  However, our *hardware* only can support 128K routes which includes unicast, multicast routes, "fastdrop" entries, and routes to directly-connected hosts.  Therefore, 128K of routes is a hard-limit on the SupIV.  With more than 128K routes, hardware forwarding is disabled and default back to process switching which is not recommended because it can be very slow.
On the Cat6500 SupII, with 512M of DRAM, we should be able to do CEF forwarding on 300,000+ routes.  Please note that the maximum number BGP routes supported is inversely proportional to the number of neighbors you are peering with.
Here is an example related to the number of BGP routes supported on the Cat6500, PFC2/MSFC2 w/ 128MB of memory.
Neighbor - Routes
1 - 97000
2 - 90000
3 - 85771
4 - 82467
5 - 77376
6 - 71235
7 - 69000
8 - 64207
9 - 61729
17 - 44528
With 512MB of DRAM on the 6500, we can assume that the number of routes supported is quadruple based on the table above.
Please let me know if you need additional information.
Ted Vo, CCIE# 11323
Systems Engineer
Bay Area Public Sector
Office Phone: (415) 371-2419

Pager: 800-365-4578