Hi CM41,
Are you only a back sleeper or do you sleep in other positions as well? What is your experience in the other positions you sleep as far as pressure relief and alignment?
As C1981c memtioned … there is “primary” support which is the support that comes from the deeper layers which “stops” your hips and pelvis from sinking in too far and there is the “secondary” support which comes primarily from the upper layers of the mattress and fills in and helps maintain the recessed gaps in the spine. This may be lacking in your mattress although 4" for a back sleeper (2" 21 ILD and 2" 28 ILD) is usually fine so it may also be an adjustment period from what you were used to sleeping on. You may actually be getting “better” secondary support (your lumbar curve as more resistance under it) and your body needs some time to get used to it.
Post #2 here has some suggestions that may be helpful. I would first tend to wait for 30 days or so (especially if your mattress still feels firmer than the one in the store) and you could even try walking evenly across the surface to speed up the initial break in period. Once it’s clear that your experience is likely to be the same in the long term … then a topper could help if the issue and “symptoms” point to a comfort layer that needs a little extra thickness and softness.
Phoenix