I'm sorry if I got that wrong: to me, four nights means three whole days. If you really have five days, then Potsdam may well be a good option for one of them, but do read about it first so you can see if you think you'd like it.
Park Sanssouci is a huge and beautiful park, with several palaces and several other interesting buildings, and the city itself has much that's worth seeing - although, if it's a nice day and you want to explore the park and the palaces, you won't really have time for the city.
And yes, do read Things to Do - and try to give us more of an idea of your interests, so that we can offer specific suggestions. Not everyone is interested in the same things, so "What to do in Berlin" has many different answers.
Edited: 7:22 pm, February 04, 2013