Hi Rtved.
I am a bit of a bookworm myself and I am glad to have company! …Welcome to our Mattress Forum! 
I can certainly see that you’ve been doing a bit of avid reading on our site as you are asking good questions.
You are correct that the two 2" layer would respond a bit more independently from each other but using two 2" slabs instead a single 4" solid piece of the same foam type (that is exactly the same ILD and density) would make little if any practical difference. In theory the 2 separate sabs would respond independently and they could feel a little softer than a one-piece slab of the same thickness but most people can’t feel any difference at all provided of course that both foams are the same type (density and ILD) and that there aren’t any cutouts or surface modifications such as the layers being convoluted. You can read a little more about some of the considerations of using multiple layering vs one slab in post #2 here.
All the layers of a mattress compress simultaneously, not sequentially, and they will each compress to different percentages of their thickness depending on their position within the mattress, the firmness of each layer, the compression modulus of the material, the thickness of each layer, and the compression force that they are exposed to. Post #4 here has a bit more about the different specs that can affect how soft or firm a foam feels. The two most important of these would be the ILD and the compression modulus although there are also other foam properties that will affect the “feel” of a foam material as well outside of just its softness/firmness that are more subjective.
I hope this gives you a bit more food for thought and satisfies your reading appetite.
Let us know if you have more “queries” and I or any of the Expert members will be glad to assist you.
Phoenix