iirc, Apple’s technology doesn’t really identify unique fingerprints. Odds are about 1/50,000 that any fingerprint is identified by Apple as unique. So there is a very small chance your phone could be stolen and the thief could enter a fingerprint (not yours) that unlocks the phone. Meaning that Apple is working from a set of ~50,000 fingerprints, one of which is linked to yours.
A 4-digit code would mean that a hacker would find your code by entering no more than 10^4=10,000 different entries. 6-digit code would cap that work at 10^6=1 million entries.
Face recognition ups the encryption power to something like 1/1,100,000 or thereabouts. Larger set of facial patterns, and an improvement on a 6-digit code.
The real advantage, though, is that facial recognition complements the move to AI. And you can morph your face using emojis. Let’s not forget the emojis.