Hi RickNY,
One of the most common “complaints” about memory foam over the years (outside of temperature) is that its slow response can feel like you are “sleeping in sand” and make movement on a mattress more difficult. Many of the newer generations of memory foam have a faster response than they used to which can help with the motion restriction on a memory foam mattress.
There are many manufacturers that use 4 lb memory foam in their mattress (Tempurpedic being one of them) because they can be more breathable and have different properties than many higher density memory foams and these types of design choices are just part of the “art and science” of mattress theory and design and how different mattresses are designed to have a different “feel” as well as fit inside different budget ranges. There is more about the different properties that can be formulated into different types of memory foam in post #9 here and in post #8 here. Many people prefer faster response memory foams while other prefer the more “traditional” memory foams that have a slower response.
Phoenix