I was a teenager when the 40 series came out and was well-studied on these tractors. I remember reading the brochure and it did say that the 4040 was indeed introduced as a "detuned" 404/4230, set to provide a "rock-solid" 90HP all day long. In much the same way the 4240 was now a 466 producing around 110HP.
We had a 4230 and it produced 104PTO horsepower with no mods. Never got a chance to run or compare with a 4040.
I"m sure you can turn up the fuel a bit and get it to 4230 levels of HP...100-105PTO HP with no other changes.