Lots to see, you won't do it all in four days.. you'll be back.. (we've been back every year since our first trip). Some photos
here, just shout if you want to know locations (e.g. the grafitti).
Get a four-day pass for the S-/U-bahn tram and bus, if you do more than two trips a day it pays for itself. Don't forget to validate it before the first use.
The one must do on my trips to Berlin is a meal on the Fernsehturm, get there about a 1-2 hours before sunset, get the lift to the top, have a quick view of the city and then quickly join the queue to get a table on the rotating top deck. The food is very good, the prices are pretty good, and they wo't rush you off the table so you can be there all evening and watch as the city lights up below.
I think
stueaston is thinking of
Tacheles, which I think is back under threat (again).
There's just so much to see on a first trip.. a walking tour taking in the Alexanderplatz, the Reichstag, Holocaust Memorial (look out for the smaller gay/lesbian memorial on the other side of the street amongst the trees), the new parliament buildings and the new Hauptbahnhof.
Friends joined us on one trip, and they had a day chasing down film locations from
Run, Lola, Run. Make sure you watch the Bourne films before you go
Freidrishshain is one of the less gentrified of the former eastern districts, it has a lot of small boutique shops selling fashion, accessories and student-designer type stuff. A lot of the small boutiques don't open until evening and will stay open late as the owners may be students and the shops are used almost like living rooms by groups of friends. This is also one of the best districts for a good cheap meal - Simon-Dach-Straße is one of the best streets to eat on.