Four corners is the shortest pattern in bingo. Mark the four corner squares of the card and nothing else, and the first person with all four wins the round. That is the whole rule, and the reason it is the pattern most hosts open with.
What counts, and what does not
Only the four corners. Not the edges, not the middle, and not the free space. A player who has the four corners plus half the card still wins at exactly the moment the fourth corner is called, and a player with everything except one corner has not won at all.
Order never matters. The corners can be called first, last, or scattered across twenty minutes of play. All that matters is that all four have been called by the time somebody shouts.
The free space question
This comes up every single time, so it is worth answering plainly: the free space does not count in four corners. It sits in the middle of the card and the pattern never touches the middle. Patterns that do cross the center, the letter X, the plus sign, lucky 7 and blackout, get a square for free. Four corners does not, and does not need one.
How long a round runs
Two to four minutes on a 5x5 card. That is short enough that four corners is almost never the main event. It earns its place as the warm-up: it teaches the room the rhythm of calling and marking, it gives somebody an early win, and it takes so little time that nobody minds losing it.
For a room of small children it can be the whole game. Four squares is about the right amount of concentration to ask for, and a 3x3 card makes it shorter still.
Running it well
Announce the pattern before the round starts. That one habit prevents the argument where somebody marks a full line and expects to win. If you are running several rounds, say up front which pattern each round uses, or put a printable pattern chart on the table and point at it.
Keep the call sheet. When somebody shouts, you check their four corners against the numbers or words you actually called, and a written record turns a possible argument into a five second check.
When four corners is the wrong choice
It is too short to be the main event of an evening. If you have set aside an hour and you open with four corners, you will be three rounds in before anybody has settled, and the wins will feel arbitrary because almost no skill or attention is involved. Save it for the opener and the closer.
It is also a poor choice when the prizes matter. A pattern that resolves in two minutes rewards whoever happened to be dealt the right corners, and while that is true of all bingo to some degree, a longer pattern at least spreads the luck across more calls. If somebody is walking away with something worth having, play any line or better.
And it is the wrong choice if you are trying to teach the game. Four corners never touches the middle of the card, so a new player finishes a round without ever having scanned a full row or noticed the free space. Any line teaches more in the same few minutes.
Variations worth knowing
Four corners plus the free space. Some house rules count the center as a fifth square, which changes nothing, since it is already marked. It is worth mentioning only because a player may ask, and the answer is that it makes the pattern no harder.
Inner corners. Instead of the four extreme corners, the four squares diagonally adjacent to the center. It takes a little longer, it looks like a small diamond, and it is a good way to get a second short round out of the same idea without repeating yourself.
Postage stamp. A two by two block in any corner. Four squares again, but clustered rather than spread, which makes it feel quite different to play and takes slightly longer because the squares must be adjacent.
Cards to play it with
The bingo card generator prints unlimited unique cards free, with words or numbers, so every player gets a genuinely different card. The full pattern chart has four corners alongside every other common pattern, and bingo rules and winning patterns covers how a round is scored and the house rules worth agreeing first.