Hi Ginger,
Viscoelastic or memory foam can be good or not so good quality but it is a slow response material which feels and performs very differently from the latex you were mentioning in the previous post which is a fast response material.
Better quality and more durable memory foam is @ 5 lbs and up although there are many mattresses that use a mid quality/durability 4 lb memory foam because it can sometimes be a little more breathable, faster responding, and less temperature sensitive depending on the memory foam formulation and some people will prefer this “feel”. There is more about the pros and cons of memory foam in this article and more about the many variations of memory foam in post #9 here.
Ultimately the choice between slow and fast response materials in the comfort layer is about personal preference but either way it’s important to make sure that the materials are good quality and appropriate for the price of the mattress no matter which they are.
Phoenix