Quadcopter: would putting 3 blades on the same level and the other blade below the first three work?
There are designs with overlapping blades. It's all a tradeoff on space, structure, clean aerodynamics, efficient aerodynamics, etc. This is the Malloy Hoverbike, for example.



So you can definitely have them at different levels, and you can even definitely have them overlap. If the 3 rotors' flow is not involved substantially with the 4th rotor's (not too big a difference in height), it should be no big deal.