Hi ddcfri,
It’s not really possible for me to know which layers would work best for you because there are too many variables. When you are doing mattress surgery (see post #2 here) or building your own mattress (see option 3 in post #15 here) then trial and error is always one of the biggest parts of the process. I also don’t know what your frame of reference is or what the design goals of your mattress are relative to a specific mattress. While it won’t be completely accurate because of all the differences between different polyfoam and memory foam materials … it’s usually a good idea to have one or several mattresses that you’ve tested that can act as a reference point for what you are trying to build then you can use them as a guideline to choose the materials for your own mattress.
If you are going only by theory without a reference point and trying to keep things simple then if you choose a 3" layer of 4 lb memory foam (and 3" is the least I would choose initially) in the hopes that a single layer of softer memory foam will be enough and it’s too firm for you then adding more softer 4 lb memory foam on top would give you a lot of 4 lb memory foam in the comfort layers which may be more risky both in terms of durability and in terms of alignment. In this case I would tend to build from the bottom up and start with about 3" of 5 lb memory foam and then add to it if you need to based on your personal experience on the mattress unless you are confident from any local testing you have done that a mattress that only uses a layer of 4 lb memory foam on a similar polyfoam support core has a reasonable chance of working well for you.
I would also tend to use a mattress cover to keep the layers together and would probably try and re-use your current cover rather than a mattress protector which is really meant to perform a completely different function (protect the mattress rather than being part of the mattress itself) although it will do in a pinch if there are no funds available to use anything else.
Phoenix