I'm running 4120 on my rev c rns. This adds the radio text feature. I don't think there are any notable new features between 3800 and 4020, it's just that you need to install the 3000 series firmware to repartition the disk before continuing onwards to the 4000 series.
I think 4120 is as far as you can safely take a rev c rns, I've read a few threads elsewhere of people trying to install 5000 series firmware and getting lots of problems.