I need to put up a fence for our puppy.

I plan on 4x4 PT posts, 4 feet above ground, 2 feet below.

I'm getting conflicting advice on concrete vs gravel vs just tamp the
dirt really tight. One expert even recommended concrete in buckets.
There seem to be several successful methods.

I want the posts to be removable eventually, for repair or moving the
fence area. So if concreted in, I'd want them to not be stuck to
it.

What do you think?

And yes, I know to call Miss Utility before digging.