Four is the most you can make a realistic list with. I faced a dude who brought five and Rieekan once, and it was okay, but it ends up with a lot of really exploitable weaknesses.
If you're looking to run an MC30-centric list that's not ridiculous, 3 will get you there with some back-end support from CR90's and squadrons, while still giving you lots of room to build the list as you want. Almost any admiral will work well with this archetype, and it will fly very differently based on what upgrades you prefer to take.
Four is really a "let's see if I can exploit this one weakness in the game for all it's worth" gambit. Which, incidentally, works amazingly well and is basically unstoppable (:D). But it's also super unforgiving, too, both in terms of how well you fly it, and in terms of luck.
You really have to make tough choices in list building and strip everything down to bare bones to get an effective list with four. I've been running the following completely unchanged since... well, since I posted it in mid-October. Just won San Antonio Regionals with it a few weeks ago. I love it, it's lots of fun to play... and it's just about time to retire it.