By default PokerTracker 4
moves your processed hand histories to your processed folder and you can reimport those files into a new database:
C:\Users\SOME_NAME\AppData\Local\PokerTracker 4\Processed
To manually import hand histories from your hard disk select "Get Hands From Disk" after clicking "Play Poker". Choose "Select Directory" to import all of the hands found in a folder, or "Select Files" to only import specific hand history files that you choose. Depending on the number of hands you have this import may take some time. You therefore would want to schedule this to start at the most convenient time. In other words I would not start this import if you have important poker sessions coming up as you cannot import manually and use PokerTracker 4 on live tables at the same time. If required you can break the import up into smaller chunks/folders.