Like previous poster said, it can be hard to get into the tents....you need to be there early in the morning to get in! After a number of hours boozing there are rollercoasters etc in the fields beside the tents which can be good craic if you can stomach it!
Munich is a nice city so I would give it 5 days as you should spend one day at least going to the city centre and doing a walking tour/bike tour. There are other beers halls/beer gardens dotted around - there's one in the English Gardens (their Pheonix Park).
Good to mix it up also and do the beer fest for a part of the day and head on to a night club later. But if it's your first time, you'll prob spend most of your time in the tents. Weekends are more busy at the tents as the locals are off work....
There's an area called Kuntspark/Kultfabrik where there are a number of bars/night clubs in one area - can be good on certain nighhts - e.g Sat night.
Nachtgallerie was another club that we went to before which was good. Hotel/hostel people usually have good advice on which club is good.
Bouncers in the city centre can be picky at times with foreigners so Kultfabrik could be option as a back up to continue the night. The tents from what I recall close up before 12 at night (possibly earlier). The beers are the best....and deceptively strong. Also, you get them in the big glasses so you are drinking more than you usually would.....pacing things out makes sure you enjoy the whole day(s)!
Should try and get it all booked asap as flights go up and accom gets snapped up....all the Aussies/Americans in particular descend on Munich at that time....not to mention the Irish!