Is PostgreSQL installed on your computer? If you've been using PT3 then everything should be set up OK, but check the Services window (click Start and type "services") - do you see PostgreSQL listed? Is it started? If not, try starting it and then run PT4 again.
If PostgreSQL is running and you still have problems please attach the PokerTracker4.log file to a
Support ticket and post the ticket number here. The file is in the "C:\Users\yourname\AppData\Local\PokerTracker 4\" folder, accessible through the PT4 "File" menu.