There is a company called Tru Angle(I think). They make a sharpener that is a block of wood with 2 files attached at the proper angle for different broadheads, whether it be 2, 3 or 4 blade. I don't know if they are still in business.
Why a single bevel on a 4 blade? I thought the purpose of a single bevel was for a two blade to aid it through the body by cutting the direction of the arrow rotation.