Please reproduce the problem and attach the PokerTracker4.log file from C:\Users\yourname\AppData\Local\PokerTracker 4\ (which you can access from the PT4 File menu) to a
Support Ticket so that we can see what is going on.
While the conversion is running please make a copy of the "temp import" folder which appears in the same folder (C:\Users\yourname\AppData\Local\PokerTracker 4\). That contains the files exported from your PT3 database and may be needed for further investigation once we have your logs. That folder is deleted at the end of the conversion, so if it crashes it may still be there, but otherwise you'll need to copy it as soon as the conversion reports that all the files have been exported from PT3. (Just control-drag "temp import" to a blank bit of space in the same Windows Explorer window and it will make a copy there.)