Hi amr2625,
Your own personal testing and/or experience is the most accurate way to know how suitable a mattress is for your specific needs and preferences. Outside of that … more detailed conversations (such as the ones you’ve had) with a manufacturer is the best way to choose and the more information you can provide them the better they can help guide you (see mattress firmness/comfort levels in post #2 here). As you have discovered, different people (even when they are all “experts” at what they do) will have different suggestions based on their individual experiences and on the specifics and level of detail of the information that is exchanged in that specific phone call (see the previous post #8 here that I linked). There are so many unknowns and variables that there is no “formula” that can predict how any specific person will feel or interact with a specific mattress that can possibly be more accurate than their own experience.
In the end … suggestions from an online manufacturer are really “best efforts” based on the averages of many customers and again only your own experience when you sleep on the mattress can confirm that their suggestions are the most suitable for your specific needs and preferences. This is why your options after a purchase can be an important part of the “value” of an online purchase (and all of the mattresses you are considering have free return options so the risk of a less than ideal choice is much lower).
The mattresses you are looking at all have a rough “equivalent” in the Tempurpedic lineup so careful and objective testing of the equivalent Tempurpedics will also give you a very good idea of the level of comfort and support for each mattress you are considering and I would try to find the equivalent Tempurpedic model so you can use your own testing experience as a reference point for the suggestions you are receiving.
While 4 lb memory foam is less durable than 5 lb memory foam … at your weight that shouldn’t be a real issue but of course that will also depend on all the other factors that are involved in the durability of a mattress (see post #4 here). Slightly less durable materials that have specific properties that provide you with better sleep is a worthwhile tradeoff when it’s compared to a slightly more durable mattress that isn’t nearly as suitable for your specific needs and preferences. None of the mattresses you are considering have any obvious “weak links” and at your weight I wouldn’t be particularly concerned with the durability of any of your choices and would be more focused on the suitability of the mattress for your specific needs and preferences so you can make the best final choice for you.
Phoenix