BarleyMan mentioned some great day trips for first-timers. Another favorite of ours that BarleyMan did not mention is Kloster Andechs; a wonderful old monestary with great views, great food, and some good bier as well.
If you are planning four days in Munchen, I would suggest only two day trips though. Depending on your arrival date/time, Friday is usually a "get situated" day, which usually ends up with us at the Hofbrauhaus for most of the afternoon and evening. You probably would want to spend one of the days just wandering around Munchen and seeing some of the local sites; Marienplatz, Glockenspiel, and some of the local bier gardens like Augustiner Keller and Viktualienmarkt. Of course there is also Englischer Garden with its 3 huge bier gardens. You just need to have a general plan and be prepared to do something different at the last minute.
Tickets for the tents ARE NOT REQUIRED. While there are reserved seats that can be purchased, these reservations technically go on sale in January, and they are almost exclusively bought up by corporations, locals and travel groups. In the past we have been able to get reserved seats once or twice through other contacts, but it is not something you can count on. The fun part of Oktoberfest, especially for first timers like you, would be to just go to a tent, and wander around until you can squeeze into a table. These are the tables with the most party oriented people, and gives you the opportunity to meet people from around the world with the same common interests as you; good bier and fun! There are 14 tents, some seating 10k - 12k people, so you can only imagine the fun one can have.
Two good resoureces for you:
- Semi-offical Oktoberfest site. Lots of good information, trivia and history.
- Beer Drinkers Guide to Munich. While not especially about Oktoberfest, this book guides you through many of the city's great bier establishments. We have used this "bible" since our first trip in 1993, and I suggest this be your very next purchase. Myself, BarleyMan and Einfahrt are promently pictured as Field Researchers here ---> Linky