Dave, 4-1" screws (often provided with the hood) is pretty much the standard installation for many range hoods, but as you say, they're not always put in as well as they should be. Often this is because the screw holes don't align well with the cabinet frame, and no additional blocking was added.
Re-working the screws for a better "lock" on the hood should work just fine, and you can use different screws if you feel that will help. If you carefully locate the framing, you could also drill new screw holes near the original "keyhole" openings. In either case, you'll probably want to predrill the cabinet for the screws, too, to prevent splitting. Good luck.