Need to raise the camper to clear the cab

Four inches is a lot to compensate for. I had a similar problem, but I only needed 2". I used a 4'X 8' sheet of 2" closed cell foam insulation material, which had aluminum foil on the outside. I sprayed the foil with spray-on bed liner to give it more "grip." It worked well, but I am not sure I would recommend a 4" layer of the same.

SidecarFlip has a good point in saying that raising the center of gravity and the overall height need to be carefully considered in whatever you do. In addition, you will need to consider the additional weight of whatever platform you create. Hope you find a reasonable and safe solution.