I've just thought of another interpretation of your question. The wall is on the ground and the slab is "suspended" 4 inches above ground level. I would model that with a wall from "Ground" for whatever height. I would have have a two levels "Gound" and "Ground Floor". You'll end up with a mysterious room that's 8 inches high. The "mouse gallery" or something.

Or perhaps I haven't understood your scenario.