Hi DrDonna,
There are too many unknowns, variables, and personal preferences involved to use any kind of formula to choose a mattress. The most effective approach would be to use the testing guidelines in the tutorial post to choose the mattress that was the best match in terms of PPP and then make sure that you know the specifics of all the layers (the thickness and foam densities) so you can make sure that the mattress uses good quality durable materials and doesn’t have any obvious weak links. With your lower weight 4 lb memory foam would tend to be a durable material.
Memory foam and gel memory foam will also tend to be warmer than other foam types depending on the combination of factors that are involved in sleeping temperature (see post #2 here).
The name of the mattress will be on the law tag if you are ever in doubt about which mattress you are testing.
Again this is fairly typical because so much of how a mattress feels and performs is subjective, each person will have their own opinions, and there really isn’t a formula that someone can use that is more effective than your own personal testing and experience.
More than anything I would use careful testing and “trust your body” to tell you which mattress is best for you in terms of PPP and then use the specs to make sure there are no weak links and make more meaningful comparisons with any other mattresses you are considering.
Phoenix