Hi DigitalDawn,
If it is 2.4lb it could be HR polyfoam (if it meets all the criteria for the HR polyfoam grade including compression modulus and resilience) which is a high quality and durable material but it is still a lower quality/durability material than latex and I would question its use in comfort layers this thick in a mattress in this budget range. While there are some people that prefer a lower resilience layer or quilting over latex such as a thinner layer of polyfoam, or natural fibers to reduce the springiness or bounce of latex … this is more than is really necessary or even suitable IMO and certainly isn’t justified by the prices they charge. In a lower budget mattress … 3" of HR polyfoam would be great to see and much better quality than the lower quality polyfoam that you would usually find in mattresses but this is not a lower budget mattress.
I would keep in mind that they use layers that are either 2" or 3" thick and layer thickness and the overall design of the mattress can make as big a difference in how a mattress feels and performs as layer softness. While it’s true that their designs could likely match the needs and preferences of almost anyone who prefers latex because of the many combinations of layering they make available in their different models … they don’t carry every ILD in every type of latex that you may find in another mattress (although they would probably order it on request for those who wanted it and were willing to give up the warranty in exchange for the use of softer materials that they strongly believe would be less durable. You can read more about trying to match another mattress based on a more “subjective” feel rather than matching more objective criteria in post #14 here and the posts it links to.
In general … your local testing can provide good reference points about the types and designs of mattresses that you do best with and this can be valuable in discussions with good online retailers and manufacturers about your layer choices and options but I would always keep in mind that every difference in a mattress increases the odds that what you order will be different in some way from what you have tested even if both of them match your basic needs and preferences in terms of PPP (Pressure relief, Posture and alignment, and Personal preferences). In other words different designs can have similar outcomes but will require some “educated guesswork” and translation of one design into another that can be as much intuitive as it is objective.
You would probably also need to know what the extra 1.5" was because the cover and quilting can also make a difference in the feel and performance of your mattress. Assuming that the cover and quilting was similar to what an online manufacturer also has available … then the base mattress would be a fairly common design that would be fairly simple to approximate. I would also keep in mind that it’s usually a good idea to test a specific topper with a mattress as a combination because adding a topper that you haven’t tested on a specific mattress is a variable that can be somewhat “risky” or produce some unpredictable results depending on your knowledge of mattress layering and the balance between comfort/pressure relief and support/alignment. If you can’t test the combination together (or a very similar combination)… then it would usually be better to first buy the “base” mattress and then use your actual experience to decide on which topper would be best based on the guidelines in post #2 here and the posts it links to which may be more accurate for your personal circumstances than a “theory” that you haven’t tested in person.
In the end … if you are considering buying a mattress online that is not the same as the one you have specifically tested … nothing will replace a more detailed conversations with a manufacturer you are considering that includes all the information you have available about yourself and your testing to decide on which of their mattress options may be the closest to your specific needs and preferences. For most people … trying to use the sometimes incomplete “specs” they are able to find to do this on their own can often be more confusing or even overwhelming than helpful. Good manufacturers already have the experience and knowledge about mattress construction and the details of their own mattresses and how they may compare to other designs that you would otherwise have to learn and which can take a great deal of time, effort, study, and experience to try and do on your own.
Connecting with “experts” is much easier than trying to become one 
Phoenix