Hi jhatfie,
Polyfoam is a “fast response” material so if this is the top layer then the overall “feel” and response will be a combination of the top layers and will “shift” the feel and response slightly away from the slow resonse of memory foam and towards polyfoam. the firmness will also supply the hand feel or surface feel of the mattress and would mean you were a little more “on” the mattress than “in” the mattress. Every type of material has it’s own unique combination of characteristics and in different mixes these characteristics are combined to create the “feel’ and performance of the mattress. Having 2” of polyfoam for example could change the feel significantly from having just an inch of firmer polyfoam on top. While it’s more common to have memory foam itself as a top layer because most people who buy memory foam want the “feel” of memory foam … hybrid comfort layers like this are also not uncommon either. For example … one of our members Rocky Mountain Mattress has several mattresses that uses this type of layering (polyfoam in the quilting layer over the memory foam).
Some of the better options and possibilities I’m aware of in the Seattle area are in post #2 here. If you have tested this type of mattress and like how it feels over all the various types of memory foam itself (and there are many types of memory foam with diferent types of response but they are all in the 'slow response" group) … then it would be a matter of calling various retailers or manufacturers on the list and asking them if they have any memory foam mattresses that have a thinner layer of either polyfoam or latex (both fast response materials) over the memory foam or in the quilting.
I think this would probably be a good idea and could make a difference yes. 4 lb memory foam is not as durable as higher density memory foam but it also has a feel that many people like (it’s sometimes but not always softer, more breathable, and faster responding than most but not all 5 lb memory foam). I would tend to minimize the use of 4 lb memory foam or use thinner layers at higher weights or at least be aware that the “feel” you are looking for may be at the expense of durability. Each layer is also affected by the layers above and below it so using higher density memory foam under it can make a difference although using higher density memory foam over lower density may have a bigger effect yet (it would sheild the lower density foam below it from much of the mechanical and durability stressors it would be subject to but may not feel as good for many people). You can see some of the many variables that can be manufactured into memory foam in post #9 here along with post #8 here.
Overall there are too many variables to make the type of 'black and white" assessments that most people would rather have (including me) and everything usually depends on several other factors in a complex “algorithm” of interacting influences. This is why local testing can be so important (it bypasses many of the unknowns) or testing a mattress that you know is similar so that you can eliminate most of the surprises. Without this if you are looking at an online purchase … then the knowledge and skill of the person you are working with to help you make the most suitable choice or any return or exchange policies in case of “mistakes” may be a significant part of the “value” of a mattress outside of the materials themselves.
Phoenix