All 747-100/200/300 series had the option (attachment point installed) to carry a fifth pod.
If an operator wanted to use this option, Boeing provided the amended OM
(after paying the required fee).
AFAIK only the PW
JT9D series and the RR
RB211-524 series were certified for fifth pod operation at the 747 classic aircraft.
engines (-50E/E1/E2 and -80C2B series) were certified for fifth pod operation. Both GE engine types were designed to be split for (lower deck) transportation and all early 747/CF6 operators (KL, AF
, etc) had 74M and/or 74F in their fleets, able to transport a spare engine (no splitting required) at the main cargo deck.
I have no knowledge about fifth pod operation at the 747SP.
The 747-400 series were also sometimes used for fifth pod operation. Known users are QF
(both RB211 series). But at that time (end of the eighties of the last century) almost all spare engines are already shipped with 74M, 74F or other wide body freighter aircraft.
The 747-8 series has not been certified for 5th pod operation. Also AFAIK no attachment point and re-enforced wing structure , inboard of engine #2, has been included in the new wing built up. ( for weight saving reasons ?).
[Edited 2012-06-30 06:02:01]
Operating a twin over the ocean, you're always one engine failure from a total emergency.