3v3 kind of works on Summoners Rift, but it's comparatively harder to gank, as one tends not to forget to call MIA when the only hero in their lane is gone. Also this version of playing really hurts junglers as they can't jungle without leaving a lane wide open.
4v4 on Summoners Rift works just fine in my opinion. Nothing to really report on that.
Constraints should be decided amongst your group of friends I would think. At Gaming.SE, this is considered a opinionated topic and our opinions might differ greatly from your friend's. You may want to use a "no top-lane" rule in a 4v4 I suppose, if for no other reason than to force two heroes to a lane and help emphasize the co-operative aspect of the teamplay, but it really is something you should discuss with your friends rather than here.
Being that you're amongst friends, one aspect I would strongly suggest is to use the draft picking method that is used in ranked games. This will allow each side to choose three champions to ban, then allow each side to choose champions in turns so counter picking strategies can be used. That way you get good training for future Ranked Games, while playing with a group that won't flame you for making a mistake or trying out something new.