Unlocking/forced unlocking is not a 4th amendment issue, but a 5th amendment one.

The 4th amendment would protect you from them seizing your phone in the first place for no good reason, but would not protect you from them seizing your phone if they believe it has evidence of a crime.

Regardless, it is not the thing that protects you (or doesn't, depending) from having to give or otherwise type in your passcode/pin/fingerprint/etc.