Harmon rates their 4 x 3.75" pads at 40lbs, then notes "when one won't do, use two."
However, I think the weight limit may have to do more with the sag factor of the panel fabric than the size of pad. The ProPanel fabric is not attached at the top, it is merely stretched tight around the sides. That may put constraints on where the pads can be used, since there would be less sag for heavily loaded pads attached near the edges of the panel, than for pads attached near the center.
So there you have it. An expert opinion from somebody who has never used the product.
EDIT...oops, it looks like I have some very old model ProPanels, maybe circa 1998 or so. My how time flies. Looks like the pictures of the new ones show a seam at the top. So maybe sag is not such as issue any more. I feel so old, sometimes.