The purpose of a proportioning valve is to allow full pressure to the front and to reduce it to the rear. It can be more complicated but that is the general function of them.
4 wheel drum cars had NO proportioning...they were designed so that the wheel cylinders at each end were engineered with "natural proportioning". Of course, the engineering was not exact so you'd get rear wheel lockup sometimes but the goal was to get the car's equipment as close to 60/40 as possible, front to rear.
4 wheel disc systems in NON stock applications may benefit from a proportioning valve ONLY if rear wheel lockup is detected. You do not want the rears to lock up before the fronts because that condition usually leads to a spin out even with a good driver at the wheel.
If NO rear wheel lockup exists, a proportioning valve is as pointless as traction control on a slant six Chrysler Imperial. It will never come into play.