Did you know a 747 could carry an additional engine? The 5th engine is not working: it is carried as cargo underneath the wing to transport it from one location to another. This special way of carrying the engines is mainly used to transport an engine to an already broken down aircraft at a non-base airport. The second 747 can fly in a new engine, after which engineers can replace the engine on the broken down 747. The broken engine can be carried back in the same way to the home base airport.
Due to the drag, the performance of the aircraft is quite heavy impacted. Crews must plan additional fuel stops to account for the added fuel burn.
See for more information and a very nice description of this process this blog from Flightradar24.
This system is also how Virgin Orbit carried their rockets up to launch altitude.