Ok, scenario is thus.
Aircraft is landing or taking off from an airfield and one engine fails. Single engine has a hope of gliding down, 2 has a strong posibility, and a 4 can go around and land safely (this is obviously dependant on altitude / attitude but in most cases there can be a happy outcome).
If this Quad Tiltrotor does the same, it will have it's 4 engines at an angle to provide lift as it will be going too slow for it's stubby wings to provide enough lift (also to avoid ground strike from the rotors).
So if they are all providing lift, take out one engine / rotor and it is like a leg suddenly dissapearing from a table. I would be surprised if any pilot or automatic system could find a way of reacting to this without the worst happening.
This system would also be likely to be used to land in 'hot' areas so the chances of losing an engine or ortor would be increased.
Also if this thing loses an engine in forward flight at 10'000ft, if it does continue to fly, it would provide a bit of a dilema to the pilots as to just how they would get it down.
If you feathered all rotors to provide 0% lift, the landing would be pretty hot and fast, probably a bit too much?