I had many jump seat rides on Trident 3s in the 1980s in Europe and on the Manchester Shuttle. I was told by crews that the engine had either been disconnected or even removed when the 3s went in for the wing cracks to be fixed.
Certainly, in over 60 flights on 3s, many up front, I don't recall the fourth engine ever being used. Also the engine produced a very distinctive howl, rarely heard.
Did WZK still have the RB162 when it was delivered to MAN?