I needed a topper for a combination mattress/boxspring. From reading the reviews, I chose the 4" memory foam thinking it would be best for my back. However, I find that I sink in so deeply that it is EXTREMELY uncomfortable--especially when I sleep on my side. Once it expanded to normal size (Cal King), I felt it was IMPOSSIBLE to return since I could NEVER get it back into a box. So I'll put up with it until I can afford to purchase something else. I think, in my case, I would have done better with a 2" topper.