Hi jovan,
I’m saying most people WOULD notice the difference in firmness between a 36 ILD layer and a 44 ILD layer under a 4" futon.
There is more about the different type of firmness/softness in post #15 here and the firmness of the bottom layer would be about the “support softness”.
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
Phoenix