Using 4 code numbers as opposed to 6

If what Mr. Finch has suggested does not work, then you might be referring to the PIN that Apple sends when you login to iCloud or Apple. This is different from your device passcode (which you said is 4 digits). The 6-digit PIN code is sent to one of your trusted devices.

See Two-factor authentication for Apple ID - Apple Support

Your passcode is 6 digits if it displays 6 spaces, unless you changed it to 4 digits in Settings/FaceID or TouchID/Passcode.

If you don’t know your 6 digit passcode, If your device has iOS 15.2 or later installed, you can reset it without a computer; check out this link→How to reset your iPhone, iPad, or iPod touch when you forgot your passcode - Apple Support. Otherwise you will need a computer:

See this Apple support link→If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

Or watch this companion video ⬇️

After restoring the phone using one of the methods above you can restore your most recent backup.

Thanks, rom.ph, good suggestion.