swingset beam (8') 4x4 vs 4x6?
I would recommend that you use the 4x6 timber and place it so that the 6" dimension is vertical. 

Make sure to also plan for some additional material to make some triangulation bracing so that the A frames stay solidly in position with the overhead beam. This can be placed inside the frames or on the outside. 



It is common to put a cross piece in the A frame to which this angled brace attaches on the lower end. The use of the brace outside the frame is used when the inside brace would interfere with the swing. Use a longer upper timber to accommodate the outside style of brace.