As far as I know only the version 5 is the problem unit. It runs completely different software.
I guess an appropriate question is, what are you looking to do with the custom firmware?
I find the stock Linksys firmware to be more than sufficient for basic tasks. I have three WRT54G routers, one running the stock Linksys firmware, and the others are running HyperWRT 2.1b1. The two HyperWRT routers connect to the first using WDS+WPA so I have wireless connectivity between all routers. Saved me from having to run wires all around my house.
I want it to be able to forward ports to any address I want too.. because I think that is why it is confliciting with me not being able to open certain ports...
bought a linksys wrt54g v4 (its serial number begins with CDFA, if i am wrong please tell me) and i was wondering if they are recommendations for firmware for version 4 routers
All the hacked firmware will work with v. 4 routers.
The v. 5 WRT54G (CDFB last 4 serial number digits) don't run on Linux. Instead, they run on proprietary VxWorks OS, and the routers also have less flash RAM and RAM than the earlier WRT54Gs.