What size and thickness plastic pipe for 4x4 post?

[buckeyefarmer]

I’d use schedule 40 4” PVC and just shave your post corners a little. It would take 5 minutes with a circular saw or just use a sharp axe.
You can put PVC caps on the pipes when not in use to keep them clean and make them easier to find.

Just like this. ^
I have 4x4 removable volleyball posts in my back yard.

 

[scaredychicken]

I want to set a section of PVC or other type pipe in the ground and then be able to place a 4x4 post into the pipe.

vertically, or horizontally ?
my guess is vertically

 

[fishpick]

vertically, or horizontally ?
my guess is vertically

Yes.
The posts go in the ground. Then a cross bar goes between them from which hangs various metal targets.

 

[MossRoad]

At our first house, we had a covered patio between the house and the garage. I wanted to put in a picket fence across one side, with a narrow gate, but wanted to be able to remove it as needed to bring larger things into the back door of the house. I found square steel tubing at a scrap yard that was 4” outside with .125” thickness. That makes 3.75” inside. A 4x4 is 3.5”. I buried the tubing and cemented it in. Built the fence in place, and it was easily removable.

IF you can find square tubing like that at a scrap yard, it’s worth it. New, it’s really expensive.