Berlin is the döner kebab capital (think a tastier version of a gyro, and possibly my favorite food in the world). There are stands everywhere that all put their unique spin on it, but it's all generally the same.

StrophariaCube hit on a good general plan. I would add a visit to the Bundestag on day 1, availing yourself of the excellent free guided tour provided. Advanced registration is required for security purposes; you can find more information here. I would also recommend a guided walking tour on the first day to help you get oriented with the major landmarks of the city, so you can be better informed when you do some exploring on your own.

You could easily spend an entire day on the Museumsinsel. One piece of advice I would have regarding museums is to skip the Checkpoint Charlie museum, which seems to appeal to visitors, but is overpriced and not very interesting.

I would look into a trip out to Potsdam or Sachsenhausen on day 4.

Kreuzberg has had a little of its edge taken off in recent years, but is still a lot of fun. There is a heavy Turkish influence in the area, so if you want to expand beyond the döner, give restaurants in the area a shot.