The R4s and R6s I have worked, used 12G2, 12G3, and 12G4 versions without any problems. I could not locate anything specific that says you have to use a specific "12Gx" for a specific Release. You obviously can't load a different version backup to a different version firmware, but you can backup to any 12Gx and restore same, regardless of what was written before you backed up, since the previous gets erased when doing so.
My recent experience last November 2008 was an R6.0 with a 12G4 that died, thought the module was history. Got another R6.0 ACS308 processor, had the same symptom when I put in the RAC. So I removed the RAC and it powered up, that's when I knew something was not right. Manually programmed to get the client up. Brought the "defective" ACS to the shop and tied it without the RAC and it booted! So the only RAC I had spare was a 12G2 in an R4.0 ACS. I put it in the R6.0 and used remote access to make a backup file copy and also backed up to the 12G2 with no problems. I then ordered replacement RAC PC cards and I received 12G3 versions and no hiccups when I tested them.
...So it does not look like the version of 12Gx makes any difference, unless someone else has some input on the subject that states otherwise.
....JIM....