Mark, the chemicals needed (as in chemical reactions) for any format are proportional to the surface of the film. In your case, 4 4x5 sheets should need the same amount of chemicals as a 135, 36exp film. Obviously, it doesn't take into account any differences about the needed volume to cover the reels. The critical detail in this case is how much you can reuse the chemicals. Whenever you agitate, you put air in the solution, which inevitably decreases their useful life. That might be an assumption from my part, but I can't think of anything else.