Hi Michael1973,
After 4.5 years the foams in the upper layers of your mattress (which are the weak link of a mattress) have probably both softened and developed impressions which could certainly be a cause of joint or muscle issues if you are sleeping out of alignment. Warranties will typically cover visible impressions that are deeper than the warranty exclusion which is typically 1.5" (see the Simmons warranty here) but not any foam softening which isn’t considered to be a defect … even if you can no longer sleep on the mattress without pain or discomfort.
To initiate a warranty claim you would normally call the retailer you purchased from who will usually arrange for an inspector to come out and measure the impressions, make sure there are absolutely no stains on the mattress, and make sure you have been using a suitable foundation (these are all warranty conditions that can void a warranty). They will then report the results of the inspection so that the manufacturer can decide if you have a valid warranty claim. There will usually be a charge for the inspection. You can also measure the visible impressions by putting a straight edge over the impressions and measuring their depth so that you can make sure that the impressions are deep enough to justify an inspection.
This is quite common after a few years with major brand mattresses which tend to use lower quality materials in the upper layers of a mattress. It’s even more common that there are “virtual” impressions where the foam has softened and the mattress has lost its comfort and/or support but the visible impressions aren’t deep enough to be covered by the warranty (see post #174 here). This is the reason I normally suggest avoiding major brands completely and buying from smaller manufacturers that use higher quality and more durable materials in their mattresses.
Phoenix