The 4th ammendemnt is the one that protects your data and other possessions in general, not the 5th. The 5th protects you from being compelled to give testimony that might incriminate you, which then protects you from being compelled to offer a password to access data that could be used to incriminate you.
Since 4th ammendemnt protections are significantly curtailed at border crossings, they can actually declare on the spot that your data is now their data. The only thing they can't do is force you to tell them how to access it - but, if they can hack your password, they actually have the right to search your phone just as much as they have the right to search you luggage.
It's like this: The 5th protects your password, which protects your data, which is already protected by the 4th, but things happen, which is why we have the 5th after the 4th and the password. Agents shouldn't be able to just take your info on the spot, but if they try, you have the 5th amendment right to try and stop them. I'm not a lawyer, but I play one online.
Sure - I'm just pointing out that it's pretty well accepted that they have a right to access your data for inspection when you cross the border, just as they do your luggage. I'm not happy or supportive of this, but it seems to be a lost battle absent new legislation.
The only consideration then is that they can't force you to help them access it. The fact that it's your data is irrelevant. This matters a lot if, for example, you have a copy of your password written down with you - as they are very much allowed to use that to acces your phone. Or, even more plausibly, if your phone is protected only by a biometrics, in which case they can compel you to provide those biometrics and access the phone that way.
Since 4th ammendemnt protections are significantly curtailed at border crossings, they can actually declare on the spot that your data is now their data. The only thing they can't do is force you to tell them how to access it - but, if they can hack your password, they actually have the right to search your phone just as much as they have the right to search you luggage.