I agree with your remarks regarding the physique of Richard
Brooker in Part III, but Brooker didn't appear again as Jason in Part IV - that was Ted White (who is uncredited in the film itself, but was subsequently credited in Part VII when footage from IV popped up).
My bigger issue with this Sideshow 1/6 Jason is that, like their prior Part III, the hockey mask is simply too oval. The Part III mask was quite distinctive as a result of some odd vacuum forming process, and also because of the 3D visuals necessary for the film. It's more elongated than is typically depicted. Of all the fan repros of that mask, only one has hit the original on the head: Crash Creations' P82v3 mask. It's incredibly accurate like no other, and captures what is missing in all other Part III fan memorabilia.