Ah the legendary fith pod!!!
This was one of those idears that looks good but in fact has so many downsides to it that most airlines choose to either three engine ferry the aircraft back or to hire a freighter. The cost of the equipment and the disruption to the operation being the main ones.
I was on the 747's for four years a while ago and I never saw one done. I did hang a fourth pod on a Tristar once and I will confirm what Swedish Steve says by saying it was a long back breaking job that I would not choose to repeat!!
Hears a question ,can the 747-400 carry one?
Rgds Dr I