I'm not sure about a four poster. But this appears to have been a chambered burial mound that has collapsed - leaving behind upright pillares and the vestigial mound. It's not a stone circle as far as I can see.

I may be confusing the term four poster. If there are circles made up of four uprights then that's the wrong classification to use here, this is a burial site as opposed to a circle type worship site. I know often these things can be confusing - Maes Howe for example is suggested as being a circle before being tunred into a burial chamber but in this case I think a burial site is a better classification.