Four in a Row

How to play

1. Choose a Mode: Against the computer or Two players on the same device.
2. Against the computer, set the Difficulty (Easy, Medium or Hard) and choose what You play: Red (you start) or Yellow (computer starts).
3. Click or tap a column to drop a disc. It falls to the lowest free space in that column.
4. Players take turns. The first to line up four discs of their colour, horizontally, vertically or diagonally, wins. The winning four are ringed on the board.
5. If all 42 spaces fill up without a line of four, the game is a draw. Press New game (or N) to play again.

The tally under the board counts Red wins, Yellow wins and Draws in this browser, and Fastest win keeps your best result against the computer at the selected difficulty: the fewest discs you needed to win. Reset score clears the tally.

Keyboard controls

The seven columns form one row of buttons. Press Tab to reach the board, choose a column with the Left and Right arrow keys (Home and End jump to the edges), then press Enter or Space to drop your disc. The keys 1 to 7 drop a disc straight into columns 1 to 7. Each column tells screen readers how many spaces are free and which discs it holds, from the bottom up.

How the computer thinks

The computer uses a classic game-tree search called minimax with alpha-beta pruning. It imagines its possible moves, your best replies, its answers to those, and so on, up to a fixed number of moves ahead (called plies). Positions at the end of that horizon are scored with a heuristic: open lines of three and two, and discs in the centre column, count in favour of whoever owns them. Alpha-beta pruning skips branches that cannot change the decision, and trying the centre columns first makes that pruning much more effective.

• Easy looks 2 moves ahead and, one time in three, plays a random column instead.
• Medium looks 4 moves ahead: it never misses an immediate win or an obvious block.
• Hard looks 8 moves ahead and spots most traps several moves before they close.

The search runs in small slices of a few milliseconds, so the page stays responsive while the computer thinks, even on a slow phone.

Strategy tips

• Play the centre. The middle column is part of more possible lines of four than any other column. Opening there is the strongest first move.
• Look for double threats. Like a fork in other games, a move that creates two separate ways to win cannot be fully blocked.
• Watch the space below a threat. A three-in-a-row with an empty space above a gap is only dangerous when the space under it fills. Avoid filling the square that lets your opponent complete a line.
• Think in rows. The first player tends to benefit from threats on odd rows (counting from the bottom), the second player from threats on even rows. Late in the game, this “zugzwang” often decides who is forced to play into the other’s trap.
• Block early. Two discs with open space on both sides can quickly become an unstoppable threat.