As promised I returned to Drummore Stone Circle on my way to Drummore Castle (300m S of the stones).
The site is a natural amphitheatre with the stone circle as the focus of attention. There are only 4 stone remaining however a stone circle with missing stones seems more likely than a giant four poster.
Given the size of the remaining stones and the views from the stones it's possible to imagine this site as a sacred place for an ancient farming community, built using readily available stones.
I am planning to visit Drummore Castle hillfort soon so I shall take the opportunity to revisit Drummore to test this theory.
Canmore ID 63926 has an explanation for the curious nature of Drummore Stone Circle. Up to 1867 there was a stone circle with 9 stones. This structure was destroyed by the farmer. The current structure of four mismatched stones in a giant four poster shape is the result of displaced stones after 1867.