I find the idea that the typical 4-digit password provides any more security against an attacker dedicated enough to make a copy of your finger pretty hard to c

I find the idea that the typical 4-digit password provides any more security against an attacker dedicated enough to make a copy of your finger pretty hard to credit. You're placing a lot of weight on your "covering" ability. (There have been times I've had to try hard not to infer someone's passcode purely from their hand movements.)