If you are thinking of buying a €7 day ticket for each day of your stay in Berlin and planning to visit a museum or two (or other attractions) it might be worth considering a
Berlin Welcome Card.
I have a little over 72 hours between my arrival and departure, so a 72-hour card seems ideal. At €27.50 I only save 50 cents but I can get a 25+% discount at various museums. If you are planning to visit Potsdam it will cost €2 extra.
A 4-day card costs €31.50 but only covers 4 calendar days. A 72-hour card seems like a better value.
If you order a Berlin Welcome Card online you have to print a PDF file but you can also buy one at the airport on arrival.