Hi cub682,
Other than various types of leg extenders or risers (some of which like those in post #1 here may work) … I would probably try using 4" of the firmest polyfoam you could find (the firmer the better so it doesn’t affect the feel and performance of the mattress) and add a cover to it for appearance and put it in between the mattress and the frame. You could also try a low profile 4" foundation but this may “rattle around” a bit or slide on the frame. Some sources for polyfoam would be a local foam shop or online in post #4 here.
Phoenix