Hi Hantra,
As Jeff mentioned the easiest way to test how your mattress would feel on a non flexing foundation as opposed to the box spring that you currently have (which has springs inside it that flex) would be to test it on the floor.
OMF is one of the few manufacturers that use an actual box spring under their latex and memory foam mattresses which can change how the mattress feels vs using a non flexing foundation. Since you were sleeping well on the mattress for 4 years it’s possible that the mattress/box spring combination may work better for you than a mattress/foundation combination so I would also check the box spring by making sure that the surface is still flat and by pressing down with firm pressure across the complete surface with your hands or your knees to see if there are any soft spots or weak areas in your box spring in which case it may need to be replaced.
Unfortunately I don’t have enough information about you or the mattress to know or guess at why you would fairly suddenly stop sleeping well on a mattress that you had slept well on for 4 years but there are really 4 main possibilities that I can think of which are …
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Something in the mattress changed. Your mattress is two sided so it may be helpful if you turn and flip it to the other side on a regular basis if you haven’t been already (see post #2 here)
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Something in the support system under the mattress changed.
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Something that you were using on top of the mattress changed (such as a mattress protector or a mattress pad)
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The last possibility is that something changed in you that affected your comfort or support and which could have changed the type or firmness level of a mattress that would be the most suitable for you to sleep on (developing a health issue or a back issue for example or a change in weight among many other possibilities … some of which could be more “sudden” and some of which could be more gradual).
The first steps though would be to test the mattress on the floor, checking the box spring, and flipping your mattress to the other side.
Phoenix