Hi chip,
There are some general guidelines for toppers in post #2 here and the posts it links to.
2" would be an “average” choice that provided a “fair bit” of pressure relief with less risk for alignment than a thicker topper.
In general terms … 4 lb memory foam will feel a little softer for most people than 5 lb memory foam although memory foam can be tricky because it changes firmness and response with different temperatures, humidity levels, and time on the mattress and different types of memory foam, even with the same density, can have quite different properties and feel.
Phoenix