Yes, the 4000# payload is based on a stripped base model truck and a 150# driver and maybe less than a full tank of fuel. Add options, cargo, passengers and a hitch and you usable payload may be down to 3000# to maybe as much as 3500#.
As for the big gasoline engines, yes, they develope the power with enough fuel. About 3 years back we met a fellow out of California pulling a 17,000# 5er with a 1 ton GMC truck with the 8.1L gasoline engine. We said on a good day he might get 6.5 to 7 mpg running 60 mph. Problem was unloaded he only got about 11 or 12 at the most.
And he said it struggled on the hills.
We a pulling pretty close to 18,000# and get 9 to 10 mpg at 63 mph. Unloaded we can get about 16 mpg at 65 mph.
You could not give me a gasoline driven pick up to pull a heavy trailer.