G-spot. A 5th engine can be placed on the 747 wing inboard of the No. 2 engine. (ie. on the left hand side beside the fuselage). It used to be quite common some years ago when engines were less reliable than they are today, a tech. engine could be transported back to base or a replacement ferried to an airport without a special cargo charter. The only web reference I can find with details is the report into the Air India 747 that crashed into the Atlantic in June 1985. The aircraft was carrying a spare engine back to base and the article gives some operational details.
http://www.airdisaster.com/special/special-ai182.shtml
Don't know how often in happens now but I have never seen it, I believe the 707 could do this trick as well.