For my $.02 worth, I'd definitely suggest not merging the different forums for versions of D&D. The problem, I'd say, is that too many folks would be involved with discussions that don't relate to their gaming experience, and would end up responding "you shouldn't have this problem, and wouldn't if you weren't using your <bad> edition."
That honestly shouldn't be an issue: currently, I don't look at all in the forums for games I don't play, but I do look in the general forum. Essentially I consider that the Casablanca of forums, where you discuss general roleplaying concepts in an edition free manner, and don't come down on anyone's playstyle. I think that works quite well to discuss general concepts that carry over from edition to edition.
I don't think folks are ready to discuss 4E without venom, I really don't.