I bought this mattress topper to use in our travel trailer, underneath the existing mattress in our King bed. In our house, we have a Temperpedic, so I wanted something that would give similar comfort. I studied density ratings and decided on the 4" with a density rating of 4. Since RV beds are not sized like regular beds, I wasn't sure about the King size. As it turned out, the width was perfect, but we cut about 6" off the length. We camped for 4 nights and the difference was great. It was much more comfortable and no hard spots. The density is good for our purpose, but there is a BIG difference between this and a Temperpedic. When you press your hand into the foam, it disappears quickly. The higher the density, the longer the handprint should remain. The Pro is that because the density is less, the weight is less, too (about 35 pounds for a King) and that is better for weight control in an RV. I ordered the topper 1 week before our trip and shipping brought it to our door the day before we left. Unlike other reviews, there was no odor. We did leave it out overnight to "breathe and relax". I am pleased with the product and with the delivery.