Berlin itself could easily fill the whole 4 days imo and I am from Cologne and try to lure everyone here. Cologne and the surrounding areas (Bonn, Rhein river, Düsseldorf, Wuppertal, ...) may be worth another visit.
But if you consider going for Cologne as well: The train from Berlin to Cologne will take about 4 hours, if you take the fast "Sprinter ICE" which goes nonstop from Berlin-Spandau to Cologne.
For any more information about Cologne (we have a total of 17 Michelin starred restaurants - two 2 stars and fifteen 1 star - any much more to offer), feel free to post your questions.

My current favourites in Berlin are:
- Prism in Charlottenburg (1 star, levantine cuisine, one of our best dining experiences in the last 3-4 years, especially the service!) - https://www.prismberlin.de/english
- Dae Mon (had but lost 1 star, creative Korean cuisine, the favourite restaurant of my cousin´s Korean wife) - https://dae-mon.com/de/
- Dumpler Knödel Kitchen (traditional Czech, Italian and German dumplings) - DUMPLER: Knödel Imbiss Berlin Kreuzberg
- Maison Umamni (Izakaya with a nice twist and kind of creative menu) - https://umami-restaurants.de/
- Bandol sur mer (1 star, creative modern with a focus on vegetables) - https://www.bandolsurmer.de/en

To the list of things to see in Berlin, I would like to add the Zitadelle Spandau (https://en.wikipedia.org/wiki/Spandau_Citadel) which is a nice place to visit and explore.