Hi sbperseus,
If I understand you correctly then the specs of the mattress are as follows …
Mattress layers (top to bottom)
-Dual knitted cover
-1" polyfoam in the quilted layer (1.5 lb density)
-2" graphite latex
-1" memory foam (4 lb density)
-1" polyfoam (1.5 lb density) (was told this is to enclose the pocket coils in a box)
-8" pocket coil
-1" polyfoam (1.5 lb density) (was told this is to enclose the pocket coils in a box)
This adds up to 14" and if the mattress is 14" thick then it would include all the layers and components in the mattress.
The top 1" of 1.5 lb polyfoam in the quilting certainly wouldn’t be an issue by itself but with the additional 1" of 1.5 lb polyfoam underneath the memory foam it would be just a little more than the total thickness of lower quality/density layers that I would normally look for in the upper layers of a mattress.
Having said that … 1.5 lb polyfoam is only a little below the minimum 1.8 lb density I usually look for with polyfoam (many quilting layers use even lower density materials than 1.5 lb) and the second inch of polyfoam is underneath 4" of foam layers that are above it that would absorb most of the compression forces from sleeping on the mattress before they reached the deeper 1" polyfoam layer so both of these factors would make me a little less concerned.
While there is no way to specifically quantify how long any mattress will last for a specific person or predict exactly when they will decide to replace it because it is no longer suitable or comfortable for them (because this is the only real measure of durability or the useful life of a mattress that really matters) and because there are too many unknowns and variables involved that are unique to each person … if a mattress is well inside a suitable comfort/support range and isn’t close to the edge of being too soft when it is new (see post #2 here) and you have confirmed that it meets the minimum quality/durability specs relative to your BMI range that are suggested in the durability guidelines here then it would be reasonable to expect a useful lifetime in the range of 7 - 10 years and with higher quality and more durable materials like latex or higher density memory foam or polyfoam (in the comfort layers especially) it would likely be in the higher end of the range or even longer and the chances that you would have additional “bonus time” beyond that would be higher as well.
These specs would be better than almost all of the mattresses made by the top 3 manufacturers (Sealy, Simmons, Serta) and while I would be less comfortable with the quality and durability of the materials if you were in a BMI range that was getting close to 30 or higher … if you are well under a BMI range of 30 then overall I think it would be reasonable to expect the mattress to maintain its comfort/support for close to the lower end of the 7 - 10 year range that I mentioned or possibly a little less. If you are in a BMI range that is getting closer to 30 or higher then I would suggest you look for a mattress that contains higher quality/density and more durable materials.
Phoenix