I used my 6# pin, why does it ask for a 4# pin?

Not sure, with what you provided, just what you’re actually doing when you see a four digit code? But Apple’s current 2 Factor Authentication system uses 6 digit security codes sent by encrypted iCloud push notifications.

The older 2 step verification system uses 4 digit codes sent via SMS text.

A screen lock passcode can be either a 6 digit or a 4 digit code, or a complex alpha-numeric password.

If you have Screen Time enabled it uses a 4 digit passcode.