I have R4s on mine with loaded rear tires and heavy implements. I purchased R4s due to them being a stronger tire for my densely wooded, very rocky, heavy clay soil. And my R4s have a lot of deep cuts in them from large, sharp rocks that R1s may not have survived. I do not think R1s would provide more traction for me as the ground is hard enough the R1s would not be able to dig down deep enough so their deeper tread would be any more useful.
But I hear someone (Michelin?) is making R1s with as many plies as R4s. So if my soil was soft enough to use R1s, I would get a set of these heavy duty models.