Did you hear about the Qantas jet with the extra engine? (flickr.com)

... what the heck? makes you wonder if its a backup because they know one of the other engines is dodgy.

not that they need all four to run but still, makes me kind of nervous

All 747s (and i presume most other commercial aircraft these days) are designed to be able to carry extra engines, as you'd otherwise need to have stockpiles of engines at all airports that your planes may land, or special aircraft used solely for transporting engines.

My understanding (though this is second hand so it may be entirely wrong/misunderstood) is that switching out an engine is a relatively simple task.