(attributive) Denoting equipment and systems designed to give sufficient warning of an impending event.

(military) A warning, relatively far in advance, of a potential attack.

A bell rung to warn people of a danger. / (figuratively) A problematic or alarming aspect.

(American football) In the AFL, a warning given with one minute to go before the end of a half, after which special rules apply, eg, stoppage of the game clock after very first down.

(American football) In the NFL, a timeout and warning given to the coaches with two minutes to go before the end of each half.

(US) A warning, surrounded by a black border, affixed to a prescription drug to indicate that it may cause serious or life-threatening effects.

(Canadian football) A timeout and warning given to the coaches with three minutes to go before the end of each half; after this point, the clock stops at the end of every play. Analogous to the two-minute warning of American football.

(UK) A form of all-points bulletin circulated to international ports so that a fleeing suspect can be identified and apprehended.

(meteorology, US) A warning issued by the United States National Weather Service that conditions are ideal for wildfires.

(idiomatic) A UK Government public alert system in use during the Cold War to advise of an imminent nuclear attack. The "four-minute" adjective was not an accurate timing, but simply used as a euphemism to refer to the incoming attack as being from nuclear missiles.