Hi SleepDeprived,
There is much more than only perception involved in the design of a mattress. The first rule of mattresses is that the “best” mattress is the one that works best for you in terms of the balance between support/alignment and comfort/pressure relief or what I call PPP (Posture and alignment, pressure relief, and Personal preferences). In addition to this the durability of the materials are also a major part of an informed choice because no matter how a mattress “feels” in the short term … all foams will soften over time which leads to the gradual loss of comfort and support. If you are using 6" of memory foam on the floor or a solid foundation then the floor or the foundation would be your support layer and the floor doesn’t adapt to changing sleep positions or weights like an innerspring of a foam which is generally necessary in a mattress. For most people this would be a risky construction without having a more resilient support system underneath the memory foam … especially when the memory foam softens in the areas that have the most weight (the pelvis) over time. you can read a little more about the pros and cons of memory foam in this article.
The mattresses section of the site also includes information about some of the theory behind mattress construction and how to match different designs to different body types and sleeping positions but building your own can involve a steep learning curve and some trial and error and cost because most people don’t have the knowledge or experience about mattress design and theory to build their own unless they are putting together a mattress that is a close approximation of a mattress design they know works well for them. There is more about building your own mattress in post #15 here and in post #3 here and in post #4 here.
Thick layers of 4 lb density memory foam can be somewhat risky because memory foam tends to “allow” the heavier pelvis to sink down too far which is why memory foam isn’t used as a support material in the bottom of a mattress but no matter what theory may say your own personal experience is the most reliable guideline.
I would keep in mind though that over the course of time a mattress that uses 6" thick layers of 4 lb memory foam may not turn out to be as suitable in the long term as you hope once the memory foam softens or some of the potential weaknesses of the design in terms of support and alignment becomes more apparent over time.
Phoenix