4 lb. memory foam offers a good density compromise. It is denser than 3 lb. memory foam, making it firmer and more supportive. However, it isn't as dense as 5 lb. memory foam, which means it will have a nicer initial feel, be just a little more yielding, and will cost less.
3" memory foam is considered to have a medium thickness. It is thick enough to provide quite a bit of support for people who sleep on their backs or sides.
