Re: For some jobs; you really do need 4 engines.
The B52 is the next best thing. It can even land sideways, should a test engine 'freeze' with its throttle open, the BUFF can land with a heavy yaw application, as if was suffering from crosswinds.
And it already runs on 8 engines in 4 pylons. A whole pylon could be spared for the test engine, and the aircraft would still have 6 good engines.