Vans supplies 6 ply with their kits. Gives you some extra load factor & stiffer tire side walls. You should consider the tire sideways movement & squash rate with the softer sidewalls & how it may work with tight fitting wheel pants.
In modern tires the ply rating does not indicate the number of plys anymore. It just a rating. Generally the higher the rating the more load the tire will take. There is no reason to put more that a 4 ply on an rv.