In this corner staircase, how many cubes share a face with exactly four other cubes?

The corner cube on the ground touches only 3 cubes: the one above it and the two beside it. The cube above it touches the corner cube, the cube above, and one on each side: 4. The two ground cubes next to the corner each touch the corner cube, the cube above, the cube beyond and the cube between them: 4. All other cubes touch fewer. So 3 cubes.