It will remain questionable in the US until it reaches the Supreme Court.

IMO is clearly a violation of the 5th amendment, but the Bill of Rights in the US has been soo watered down at this point they may not even be considered rights any more....

4th amendment, I think. Regardless, given the current composition of the Court, I wouldn't hold your breath for the situation to improve.

One could make that case, but 4th allows for searches with a warrant

I would say 5th because divulging the contents of ones mind is testimony which one should not be compelled to do, the 5th profits you from being a witness against yourself, reveling a password is being a witness against yourself

Fifth: the password is testimony. It's searching their things, but the warrant issued by the court means that it's likely a reasonable search (ergo, not a violation of the 4th).