I don't know how to do it, but couldn't you use one database for all 4 sites and give each a different table prefix except the user table. Not sure how that would be done in the code, but I have seen it done somewhere else. (can't remember where now, not with pligg but another
cms). As long as the user table is the same accross the different sites it should work fine. (no extra fields for one site or another). It could be done I'm sure.