When an airplane is carrying a 5th pod, what is it and what plane can do that.?

Answer 1

Not too sure what your question means but a 5th pod on a plane would i suppose be referring to a spare engine.

"To enable easy transportation of spare engines between sites by airlines, early 747s include the ability to attach a non functioning fifth-pod engine under the port wing of the aircraft, between the nearest functioning engine and the fuselage. Photographs of planes flying in this configuration are highly prized by aircraft enthusiasts."
- except from http://en.wikipedia.org/wiki/Boeing_747

Answer 2

easy one- it is ferrying an engine to another location for repair