If you use pipe you would need a cap so dirt doesn稚 fill it up. That痴 the issue with 4 or 8?..........
Here is the pic of the pipe I cut for use with the hydrant,,,

The pipe is 8" ID, 16" tall,, and split to allow it to fit around the pipe,,
the pipe is 1/2" thick, I heated it with a propane torch so I could bend the ears up to allow clearance for the pipe.
I also heated it so that I could get the slot open enough to slip over the pipe.

After I put the pipe on the hydrant, I will fill the pipe with 57's gravel,, and maybe some landscape fabric on the top.
I want gravel around the bottom of the pipe so that it will be less "wiggly"
I added a half dozen saw slots to make sure the water will drain away,,,
I cut the pipe with a regular circular saw.
The pipe was broken from where we ripped it out of the ground with a backhoe,, that is why the end looks "ratty"