Quadcopter: would putting 3 blades on the same level and the other blade below the first three work? Most quadcopters have their propeller blades all on the same level, or in some cases, a pair of blades placed a little above the other pair. 

However, I was wondering if three propellers of a quadcopter can be put on the same level and the other propeller a little lower. Would it just make the quadcopter inefficient?
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.