Four Crosses is a portal dolmen, with three uprights supporting its capstone. It was restored by the landowner in the 1930s, who seems to have accidentally restored one of the uprights as the capstone.
Don't try to find the footpath marked on the OS map, as it is no longer there (it's now under a nursery). Instead, park in the layby opposite the nursery, and walk down to the farm turning (named Cromlech, handily). Down the lane, and turn right through the gate opposite the farm buildings.
I suppose this chamber could well be the stones referred to in the story:
Hundreds of years ago they used to keep the Collection money in the Church. One time, thieves broke into Llanfaelrhys Church to steal the money. Somehow, while at their work they were seen by passers by, who went into the church. When the thieves saw this they fled for their lives and they were followed by their pursuers until they came to the top of Rhiw, there the thieves were caught, on the road by a place called Terfyn. After catching the thieves they killed them on the spot, that was the punishment in those days for thieving. The two were buried in Four Crosses Field, Rhiw, and to show where they were buried big stones mere placed on their graves and till today these stones are called Lladron Maelrhys, but few people know of them today. It's a pity that old things become lost.