When you are talking ply I assume you are talking total ply and not just sidewall? I have the Wildpeak AT3W and mine show 7 total plys on the side of the tire.
tread - 2 plys polyester, 2 plys steel, 1 ply polyamde
sidewall - 2 plys polyester
I assume I will still be fine towing with these tires.
When tire plies are mentioned, most do not mention sidewall plies unless you start getting into the details. A 10 ply (E) tire has a strength rating of 10 tread plies, doesn't mean it actually has 10 plies. On my 275/65R20 LT tires on my 2014, I believe it had 6 actual tread plies, but some were thicker than others, making it a 10 ply rated tire.
Even though our Wildpeaks are 4-ply rated tires, they have 5 actual tread plies. I think this is one of the reasons why they are a little heavier than other tires in the same size, and I think their tread is a little tougher because of it. But that is just a guess.
The sidewall plies aren't counted when referencing a tire's load range (SL, XL, C, D, E, etc.), but guys who off road definitely pay attention to them.