Hi Barb,
I certainly wouldn’t choose a 4" topper which wouldn’t be suitable for most people but if all you need is some extra pressure relief and softness and you choose a good quality topper (4 lbs density or higher in the case of memory foam) that works well for you in combination with your mattress then your problem would be solved and it would work as well as any mattress that was suitable for you without a topper. The top layers of a sleeping system will generally soften and break down before the deeper layers (whether they are a topper or inside the mattress cover) but a suitable topper has the advantage of being replaceable when it softens while if the same layers were inside your mattress you would need to replace the entire mattress even if it is only the top layer that has softened or broken down (which is most likely). A topper can also improve the durability of the layers below it. The topper guidelines here may be useful if you decide to go in that direction and if you up with a suitable combination you would have a good quality sleeping system that is much more suitable for you for much less than the loss you would be taking on the mattress.
Of course this is only a suggestion that may or may not be the direction you wish to pursue.
If they are shredding their returned mattresses then at least you know that they aren’t re-selling it as new which is what some unethical retailers or manufacturers do. Some of them will also sell their returns to liquidators who in turn may sell it as a “scratch and dent” mattress when in fact they are used mattresses. To me this would be a big plus in the reputation of a retailer that they aren’t resorting to unethical business practices. I also don’t think it’s reasonable to expect a retailer to take back a second mattress when their policy is only one and I really don’t see how this could harm their reputation if they are transparent and clear about their policies. If a retailer was to go in this direction then the retail cost of their mattresses would need to be raised to make up for it (more than they already are to include one mattress exchange) and that could put their prices out of a competitive market range. Exceptions can quickly become the rule as soon as a retailer does this once … especially in a smaller community.
Once you are ready to start researching a new mattress then the tutorial post here is the best place to start and has all the basic information, steps, and guidelines that can help you make the best possible choice … and hopefully avoid the worst ones.
Phoenix