RE fingerprint sensor on FP3 and FP4, I’d say that performance is pretty good in both cases as long as you take care with setting up your print records.
FP4 having the sensor on the power button is not as good as the FP3 in my opinion.
You might be interested in a post I wrote about setting up fingerprints for the FP3, equally applicable to the FP4 I would say.
Once you’ve got it working properly, a good rule to follow, is: as soon as you encounter a failure and have to fall back on your other unlocking method (PIN, pattern or whatever), go immediately to settings, delete the oldest fingerprint record and make a new one. This is inevitable as our fingertips change in time, especially with the seasons.