Hi gusterbrown,
Welcome to the Mattress Forum! 
No, you do not add the layers of foam together, but evaluate each layer based upon the specifications provided. Those specifications remind me of a mattress from Worldbed Studio (I don’t think they are a going concern any longer) that was made for them by Innovative Bedding Solutions or their sister company Abad Foam. The memory foam specifications for the 4.5 lb are ok, but the 3.5 lb is a little lower than I would normally recommend, along with the 1.5 lb polyfoam, but these layers are a bit lower within the mattress, so for someone of a lower BMI the design may be a bit “borderline” (there is quite a bit of plush material in the upper comfort layers), but for someone of a higher BMI I would advise caution. You’d also want to know the density of the polyfoam base layer.
I would avoid looking too much at the “word definitions” calling a foam “high density”, and instead look at the actual empirical evidence of the actual density number. Manufacturers will call 1.5 lb poly foam “high density”, but 1.8 lb is a better starting point for a more durable foam.
I’m not familiar with research showing that simply because a polyfoam or memory foam uses a small percentage of the polyol from soy that it produces a “cooler sleeping” foam. It’s not really possible to quantify the sleeping temperature of a mattress for any particular person with any real accuracy because there are so many variables involved including the type of mattress protector and the sheets and bedding that you use (which in many cases can have just as significant an effect on sleeping temperature as the type of foam in a mattress) and on where you are in the “oven to iceberg” range and because there is no standardized testing for temperature regulation with different combinations of materials … there is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range.
Rocky Mountain Mattress is a member of this site which means that I think very highly of them and that I believe that they compete well with the best in the industry in terms of their quality, value, service, knowledge, and transparency. They are a good source for objective polyfoam or memory foam information.
If the foams used within mattresses you are considering are CertiPUR-US certified, they would meet the same minimum standard for low VOCs. Whether or not a polyfoam or memory foam uses a small amount of plant-based polyols doesn’t by itself dictate lower VOCs of that foam (most polyfoam in North America is now made with an amount of plant-based polyols). Reducing VOCs of polyfoam can be accomplished in many manners, such as reducing fugitive tertiary amine catalysts, with new polyols being introduced with built in catalytic activity, reducing the amount of catalyst package necessary to produce the foam, therefore reducing the amount of VOCs emitted from the cured foam. Also, changes in the isocyanate from TDI to an all-MDI technology have been used. While I’m not a chemist, it’s a combination of many ingredients that lead to the lowering of VOCs in polyfoam or memory foam, not just the use of a small overall percentage of plant-based polyols in the manufacture of the foam itself.
Technically, while polyfoam biodegrades, but is not biodegradeable. The finished product has crosslinks made of the molecules, which is not easily undone.
This just means that some of the fiber used in the cover of the mattress uses bamboo as the precursor for the viscose-dope used to create that fiber. Bamboo has a softer hand, but this is something used as marketing to make people think the cover is some sort of “natural” or “organic” product.
Newer generations of memory foam tend to be more breathable than previous generations, but memory foam in general, regardless of additives (phase change materials or gels) will still be quite insulating and tend to be a “warmer” sleeping product.
Newer polyfoams and memory foams are being designed to off-gas less, but there is no comparative to analyze this statement. Off-gasses less as compared to what? The more important thing to look at would be if it has CertiPUR-US certification.
Phoenix