If the older laptop has the majority of your hands then I would recommend backing up that data and restoring it on the newer laptop:
Guide: Backup & RestoreOnce that database is restored you can then import the hands played on your newer laptop from your 'Processed' folder:
C:\Users\{Windows Login}\AppData\Local\PokerTracker 4\Processed
Guide: Processed FilesTo manually import hand histories into PokerTracker 4 select 'Play Poker -> Get Hands From Disk'.