Without seeing them, I'd expect the 4 ply would probably be "softer" or have more flex/deflection under weight, which is a desirable attribute in a tire that needs all of the grip it can get. As long as your rear GAWR (axle weight rating) is less than 5600lb the lighter tires are within spec for the vehicle.
For summer tires I'd probably pick the 8 ply for durability, but for winter use I'd suggest the 4 ply and de-rate your driving speed/towing aggressiveness as needed.