Hi JustWantWhatIHad,
As you can see here … the durability of a mattress and how long it maintains the comfort and support you need is relative to each person. It’s much more likely that your initial selection was too soft for your needs than it is that the mattress is worn out or has softened more than the norm. If your initial comfort choice is on the edge of being too soft … then even though the materials themselves are not close to the end their expected useful lifetime, there may be enough softening that you are no longer in the range of having “enough” support for your body type and sleeping style even though for someone else where the same mattress was closer to the middle of the of their support needs initially the same amount of softening would be fine for many more years.
In some cases … even the initial break in period of a mattress over the first few weeks could be enough to put someone over the line where the mattress no longer provides them with the support they need. In most of these cases the “mistake” was choosing a mattress that has comfort layers that are too soft and thick to maintain good alignment for a long period of time as the material gradually softens. The other most common cause is choosing a mattress where the comfort layers are very low quality which soften or break down much more quickly than higher quality materials but this is not the most likely issue with the Cloud Supreme which uses 4 lb memory foam in the top layers.
Actually the warranty exclusion for a Tempurpedic mattress is .75" of impression (not 4") that can be measured but the real issue is not the depth of the impression which is unlikely to happen but that the material has softened enough to allow your heavier pelvis to sink down too far which can tilt the pelvis and put your spine out of its neutral alignment.
Unfortunately it’s not really effective to try to “fix” a mattress that has become too soft because anything you add on top of it will tend to just follow the dips and soft spots in the mattress which are usually the root cause of the problem. To fix a mattress that has softened you really need to remove the foam that has softened (or was too soft in the first place) and replace it with a new layer (in many cases that is firmer or thinner).
Post #4 here though may provide some partial solutions that may be helpful at least for a while.
if you are looking to buy a new mattress (which would be a shame but may be the best long term solution) … then the first place I would start would be post #1 here which has all the basic information, steps, and guidelines you will need to make the best possible choices that are more suitable for your specific needs and preferences (and easier on the budget as well).
If your mattress is showing an impression that is getting close to the .75" warranty exclusion then it may be worth keeping it for a while in the hopes that it will go over the threshold for a warranty exchange.
Phoenix