I have not done so, but I believe I can haul four. First of all, I have turned all my boat trailers into flat beds and they are all carpeted. They are 8'6" wide.

So first Wave goes upside down, second one nests inside, right side up.
Then using padded 2x4s place the 3rd boat upside down on top of the one that is right side up. Then nest the fourth.