This corner staircase of glued cubes is painted on every outside face except the faces on the ground, and then taken apart. How many cubes have exactly four painted faces?

A cube's painted faces are those touching neither the ground nor another cube. The two end cubes on the ground touch the ground and one cube: $6-2=4$. The two cubes in the second layer at the ends touch one cube below and one beside: 4. The top cube has 5; every other cube has 3 or fewer. So 4 cubes.