Hi avs,
This is normal for all mattresses but more so with foam mattresses (memory foam, latex or polyfoam) or even pocket coils which are more “point elastic” and will sink down more with the more concentrated weight of sitting where most of your body weight is concentrated in a much smaller surface area. Sitting on a mattress really has very little correlation to how evenly supportive a mattress is or how well it keeps you in alignment when you are lying down.
It sounds to me like the mattress may not have been the most suitable design for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) and you may have been close to the edge or being out of alignment when it was first purchased and a relatively small amount of foam softening both in the initial break in period and over the months that for others would still be fine may have put you “over the edge” (see post #2 here). This would likely be more connected with back sleeping which needs thinner comfort layers than for side sleeping although it is “in the range” where it would be OK for others that have a similar body type and sleeping style. It may also be that your weight distribution or body type needs firmer support layers or that you need slightly thinner or firmer comfort layers although the gel layer on the very top may have given the impression that the mattress was firmer than it really was because pure gel is firmer and the 4 lb memory foam underneath it would probably be softer in many cases than higher density memory foam and would soften a little more quickly under higher weights. The softness/firmness of the deeper support layers is more difficult to test unless you specifically test for alignment in the showroom.
Most foam mattresses don’t have or need edge reinforcement although there are a few that have it for those that prefer it if they sleep on the outside couple of inches or use the mattress for sitting. You can read a bit more about this here and here. There are both pros and cons to edge reinforcement but it is generally used more with innerspring mattresses than foam mattresses. This wouldn’t have an effect on the inner part part of the mattress but only on the feel or firmness of the edge.
Overall my guess is that your issue may have to do with the design of the mattress and how well it “matched” your body type and sleeping style than the actual softening of the foam itself (although there would be some degree of foam softening when it was breaking in and more slowly over the months since) … although of course this is only an educated guess based on your feedback and in looking at the design of the mattress.
I think the most important part is what is next and some of the suggestions in post #4 here may be helpful.
Phoenix