Definition
A UK government public alert system in use during the Cold War to advise of an imminent nuclear attack.
How People Actually Use It
The rumbling of your stomach and feeling in your bowels that lets you know that you're going to take a dump in the very, very near future. A four minute warning is also the amount of time the UK public would get between the start of a nuclear attack and the first impact.
"Christ, I've just had the four minute warning. Find me a toilet quickly or I'm going to shit myself."
Source: Urban Dictionary