Revs 1&2 use ROM set 1, and are considered to be unsuitable for the cocktail/cabaret.
Revs 3&4 use ROM set 2, and are considered to be suitable.
Afaik, you can convert Revs 1/2 to 3/4 buy simply replacing the ROMs.
There is really only one MC PCB. The revs refer to the fact that 1&3 were populated with more ROM chips. Revs 2&4 are the same as 1&3, but with the later ROM set.
If that makes any sense!
Anyway, I know you like to get these things bang on, but I have used a Rev 2 board in a cocktail and it was fine. Depends who you talk to. The only real difference is trak-ball speed, but on the Rev 2 board I used I thought it was fine.