Hi rkk5000,
As I’ve mentioned earlier … there are too many unknowns, variables, and personal preferences involved for anyone to be able to know what would work best for you based on a formula, specs (either yours or a mattress) or “theory at a distance” (see mattress firmness/comfort levels in post #2 here) so I don’t make specific recommendations (outside of the more general suggestions in the posts I linked) … especially when I don’t have any specific reference points of mattress/topper combinations that you have tried and how they worked for you.
In most (but not all) cases … 5 lb memory foam will tend to be firmer than 4 lb memory foam but there is more information about memory foam density and topper thicknesses in post #8 here which is one of the links in the post I mentioned earlier. In terms of durability … the foam density guidelines here also suggest being cautious and reducing or minimizing memory foam under 5 lb density once you are in the low 200 lb range or higher.
Phoenix