I had a builder put up a raised 10x13 post and beam foundation in anticipation of a shed placement later. I measure it and find the diagonals are 4" off. What is the easiest way to adjust the rhombus back to square? Do I add a shim piece to the 6x6 posts on opposite corners?
Assuming that the sides are parallel, since you called it a rhombus, and the framing is attached to the sides of the post as the drawing seems to show: You can definitely add shim pieces to the posts on the side that is at the top and bottom of your drawing. Depending on the load that the fasteners take, You may need to attach a block of some sort to support the end of the joist, since it won't be properly attached to the long side anymore.
You could also frame the walls square, and have them overhang the short corner.
Ideally, you should not finish paying the builder until he fixes it.