Hi Kooshball,
First of all I want to thank you for taking the time to find the correct specs and adding them to the Mattress Forum!
It’s unfortunate when consumers have to do this when it’s the job of the retailers that are selling the mattress and it’s even more unfortunate when a retailer “blurbs” out a bunch of meaningless or clearly inaccurate numbers and doesn’t even have enough knowledge t know immediately that they don’t make sense and say to themselves “something is wrong with these numbers” before they present them as being accurate.
This is also one of the reasons I appreciate manufacturers or licensee groups like Restonic because so many of their licensees are willing to provide accurate information … even when their retailers are too lazy or ill informed … or just too focused on saying anything they believe will make the sale … to find them out for themselves.
Now that you have the facts behind the mattresses you are looking at … you can make more meaningful assessments.
Durability is relative to each person, each set of circumstances, and can only really be assessed in relative terms so it’s not possible to put an absolute number on it that is meaningful. There is a great deal more (starting with a practical definition of durability) in post #4 here though that will give you some insights into the relative durability of the mattresses you are considering.
4 lb gel memory foam would be in the “mid density” range and would typically be a fairly durable material although it will soften more than higher quality 5 lb memory foam (gel or otherwise). Since this is the top 4.75" of the mattress … how long it will maintain the comfort and support you need would depend on how close to the “line” you were in terms of PPP (Pressure relief, posture and alignment, and Personal preferences). If the mattress is just barely on the edge of being too soft for you … then even the initial softening of mid quality materials may put you over the edge in the initial break in period of 30 - 90 days even though the materials themselves are not “worn out” and for someone else with a different body type, sleeping position, and preferences and sensitivities this same mattress may still be well within the range of their needs and preferences for many years.
1" of low/mid density polyfoam under this would not be a significant issue IMO because being deeper in the mattress the layers above it will absorb most of the compression forces from sleeping on it and an inch of polyfoam inside the upper layers is within the range of “an inch or so” that I would normally consider to be OK. The softening of a material that is only an inch thick isn’t generally the “weak link” of a mattress.
Finally the base foam is in the range of the minimum that I would also consider to be acceptable and while it may not be the highest quality available or of the same quality as you will often find in higher quality/value mattresses (often 2.0 lbs and higher) … it also wouldn’t be the weak link of the mattress because it’s primary function is to “stop” the compression of the heavier parts of the body and the layers above it will be absorbing most of the compression forces of sleeping.
Overall … this is “reasonable” quality/durability and it’s value would depend on how well it compared to other similar mattresses in the price range you are looking at. It is certainly better quality than you will often find in major brand mattresses that often use similar or even lower quality foams in higher price ranges.
This mattress has more of the lower density/durability polyfoam closer to the top and would not only likely be firmer for most people (depending on body type and the ILD of the polyfoam under the memory foam) but would also have a higher risk of foam softening of the 2" of polyfoam underneath the memory foam (vs 1" in the other mattress). This is because the layers that are absorbing the stresses of compression are thinner (they will compress to a greater percentage of their thickness before the compression forces “reach” the support layer". The “other side” of the durability factors here is that there is also slightly less foam to compress so that softening will have less effect on the overall feel and performance of the mattress.
Your lower weight would mean that lower density materials may be more likely to be suitable for you because they will last longer than they would for someone that was a higher weight. I would also be very cautious with the Inspire if I was a stomach sleeper (depending on how much time you spend on your stomach) because 4.75" of memory foam and another inch or soft polyfoam is a fairly thick comfort layer for a stomach sleeper and there could be a real risk of “hammocking” and the back issues that can come from this for a stomach sleeper.
So I would test each mattress very carefully and as objectively as possible so you can make sure that you are well within the range of your needs and preferences (a little on the firm side) so that the initial foam softening doesn’t put you over the edge and also make some good comparisons with your “finalists” at other retailers you are looking at to determine which mattress best fits your personal value equation.
Depending on your confidence level that your personal testing is a good and accurate reflection of your longer term experience on these mattresses … it may also be worthwhile making sure you have answered the question of … “What is my recourse if after the initial 30 - 90 days after the mattress is broken in this mattress no longer fits my specific needs and preferences?” You can reduce the odds of this happening either with more specific and objective testing (which is the ideal) or with a retailer who has an exchange policy that is low cost … as long as there are mattresses there worth exchanging for.
Hope this helps.
Phoenix