Straying slightly away, IIRC (and I do stand to be corrected on this) it's possible to ferry engines on 747s by flying them over in a 5th engine position on an inbound a/c.
There is a 5th engine mount on the port wing but its rarely used these days. Mainly because engines rarely fail and partly because if it happens downroute the aircraft are nowadays generally ferried empty with the 3 working engines back to base by a training captain so approved.