I don't know what configuration you are running for the battery in your RS4 - you didn't mention if you were running stick, side-by-side or saddle....

If you are running saddle, you will have a lot less to choose from as you probably have a height restriction and the majority of the good saddle packs are taller than the clearance you have (i.e. Reedy 4800). For bashing - the Trakpower 3200 saddle (20C I believe) would be alright and should fit. However, for about $70 on ebay, you can get an Intellect 4200 30C saddle pack, and that would be adequate for racing too. LRP recently introduced a saddle pack that may fit (check the measurements) in 4800 mah 40C.

If you're running stick, there are a ton of cheap, cheap LiPo stick packs on ebay that would probably be fine for bashing. For racing, an SMC or Thunder Power 40C pack is kind of the baseline now.