Hi KTMarty,
I would read this post and this post and this topic (about their polyfoam and sources) and this post (presumably from a past employee) before buying anything from Foam by Mail (AKA FBM and Foam Factory) or considering them as a reliable supplier … especially if the specific properties or firmness level of the foam you are buying is “application critical” and would make a significant difference to you.
If it solves the problem that you are hoping to solve then it would be but if it doesn’t then it wouldn’t.
A 4" thick memory foam topper is very likely to be thicker than you need and could be risky in terms of alignment. If you are just looking to add some additional pressure relief to a mattress that is too firm and your mattress is still in very good condition and doesn’t have any soft spots or sagging on the sleeping surface then post #2 here and the topper guidelines it links to has more information about choosing a topper that can help you use your actual sleeping experience as a reference point to choose the type, thickness, and firmness for a topper that has the best chance of success and also includes a link to the better online topper sources I’m aware of as well.
Phoenix