I'd jump straight into 4 man; there's really no need to play 3 now unless you're doing it for the campaign (who would?). Multiplayer will be full of vets and unless you're planning on buying the premium pack for 3 then you'll really have a shortage of maps compared to everyone else. Last time I played they only had 1 or 2 'vanilla' maps playing and they were mainly large vehicle ones full of jets. I've yet to come across somebody higher than like level 30 
The people still playing 3 will be a tad more experienced than a noobie turning up
