I've not been able to find the typical 3/8" pickup bed mat in longer than 8'. If you get a fitted one it will be wider than 4', at least in front and back of the wheel wells. I've not been able to find stall matting less than 3/4". Tractor supply lists 1/2" but not in my area, and no 3/8". I have looked.
I ended up using a fitted mat, cut the pieces off wider that 4' ahead of the wheel wells (as the camper does not touch there) and used them to fill in 6" at the front, shifting the mat back. There is a little customization required. You can also buy the tailgate mat, which gives you another 5' x 2' or so of material to play with.