How should I build a skid foundation for my shed?
I'd opt for using the 10 ft 4X4's with a sistered joint in the middle. You could sister the joint with a 2 ft length of 2X4 on each side, so one 8 ft 2X4 would do both skids.  Be sure to taper or bevel the ends of the 4X4's so it will "skid".