MCOGGINS:
To add to JAE’s first sentence, the load will certainly not distribute uniformly as (Load)/(4'x8'), a uniform load in lbs./sq.ft. over 32 sq.ft.
In fact, if the base of the load is very stiff (engineering judgement, experience and some calcs. come into play here) the load will distribute fairly well over the base area. Thus, you have (Load)/(6'x3'), a uniform load under the base. So, given the bending of the plywd. and stl. pl. you really haven’t gained much by adding them. You would likely do much better by putting a layer or two of 6"x6" wooden timbers/beams, 3 or 4' wide by 8 or 10' long under the load. Because of their greater stiffness the timbers would distribute the load over a greater area at less cost than the plywd. and stl. pl.