in practice you're right, but it's hard to see how that squares with the 4A.
an officer can't stop me on the street and demand to see the contents of my phone -- unless they can show "probable cause" that I was about to commit a crime (based on other evidence), or I'm already named as a suspect or POI in an investigation. So if they ask to see the contents of my phone and I delete it instead (it's a very small bag in this example, Lol) am I obstructing an investigation?
> am I obstructing an investigation
If the investigation was legit, then yes. You should just say no. If you give the phone over to them or delete data on it after they ask to see it, you are destroying evidence. Does not matter whether you are the target of the investigation or not, they think you have evidence, you deleted it.
Just say no to the police and then keep your trap shut. This is especially true for federal agents.