[murbard2]

It depends on your design. You can build it with separate engines and manage to land safely on two diagonally opposite engines.

[kenrikm]

I'm not sure that's feasible. I've been flying multi-rotors for quit some time and if you lose a motor on a quad it's nearly instantaneously on a collision course with the ground. Y-6 or Hexacopter would be more feasible from a redundancy standpoint.