If your storing hand histories in your Betfair client then PokerTracker 4 should be configured to import from here:
C:\Users\{Windows Username}\AppData\Local\Betfair Poker\data\{Betfair Player Name}\History\Data
If the folder doesn't get added to your settings (Configure -> Sites & Import Options -> Site Configuration -> iPoker Network) using 'Auto Detect' then you can manually configure the folder by clicking 'Add' and manually selecting it. If you do not see the AppData folder in Windows File Explorer then please refer
this guide.