I haven't looked closely at the NL construction, but it is similar to my Bigfoot I think. These are supported by the fiberglass sides of the box, there is not much in the middle. A 4' wide mat is fine from the wheel wells forward, but the box gets wider aft of the wheel wells, nearly filling the tailgate opening which is more like 5'. A 4' wide 1" thick mat is trying to support the floor in the middle, where there is little structure.
I would try buying another small piece of 1" and filling in between the wheel wells and the tailgate to at least the tailgate width.
I also agree with the sentiment that too thick and too soft isn't desirable. I use 1/2 plywood (wider that 4' at the tailgate) with a DeeZee type 3/8" hard rubber mat. The plywood helps bridge all the irregularities in the bed, the mat keeps the camper from eating the plywood or vice versa.