Four in a row is one move away from five. That makes every four a serious threat — but not all fours are equal. An open four wins the game on the spot. A simple four (also called a closed four) only forces the opponent to block one point. Understanding the difference, and knowing how to use each one, is the heart of gomoku tactics.
The open four
An open four is four of your marks in an unbroken row with both ends empty. It threatens to become five at either end, and the opponent can block only one of them per move.
Once your opponent has an open four and you do not have a five of your own, the game is over. That is why the whole of gomoku defence is about stopping threats before they become open fours — usually at the open three stage. See open and closed threes.
Under the blocked-ends rule an open four also wins: when X completes five at one end, the other end is still empty, so the five is not blocked at both ends.
The simple four
A simple four has exactly one way to become five. There are two main forms.
Closed four: four in a row, blocked at one end.
Broken four: four marks with a single gap, such as X X _ X X or X _ X X X. Filling the gap makes five.
Either way, the opponent must block that single point immediately. A simple four does not win by itself, but it is a forcing move: it tells your opponent exactly where their next mark must go.
Why forcing moves matter
A forcing move is valuable because it gives you control. When you make a simple four, you know your opponent's reply in advance. That lets you:
- Gain time. You place a mark that builds your position, and your opponent's reply is forced to a point that may be useless to them.
- Steer the position. You can drive your opponent's marks away from where they want to build.
- Set up combinations. A four that also creates another threat — a second four or an open three — usually wins, because the opponent can only answer one.
The classic winning patterns in gomoku — the four-three and the double four — are built from simple fours. See four-three combination.
When not to make a four
Because a four forces one reply, making it spends one of your lines. Once the opponent blocks, that line usually cannot become five. Beginners often make fours too early, for no reason except that they can. Before making a simple four, ask:
- Where will my opponent's forced block land? If it helps their shape or blocks another of my lines, the four is a gift.
- What does my four-mark build? If it creates a second threat, good. If not, it may be better to keep the three as a reserve threat.
- Is the line worth more as a three? A closed three can become a four later, when a forcing move is exactly what you need.
Under blocked ends, this caution matters even more: a closed four whose single open end gets blocked becomes dead forever, because it now has opponent marks at both ends.
Defending against fours
Defending against a simple four is simple: block the winning point. There is no choice. What you can choose is how you deal with the positions that produce fours:
- Block open threes early, so they do not become open fours.
- Watch for broken shapes that become fours through a gap.
- Before blocking any three, check where the attacker's four-mark would land, and whether it would create a second threat.
If you see that the opponent has two potential fours through the same point, occupy that point before they use it. That single prevention move often saves the game.
Counting fours in the heat of battle
In a fast game with a clock, recognising fours instantly is essential. A good habit: every time the opponent places a mark, ask "is there now four of theirs on any line through this mark, with a way to five?" If yes, block immediately — nothing else matters. Only after that do you look at threes and your own plans. This is the first rule of threat priority order.
A short forcing sequence in words
Here is how fours work together in a typical attack. X has a closed three on a row and, separately, two marks on a column that crosses the row just beyond the end of the three.
X extends the row three into a closed four. O has no choice: O must block the single winning point. But X chose this four carefully — the new X mark on the row is also the third mark on the column, turning the column into an open three. Now O faces a problem: the four was answered, but the open three on the column is new. O blocks one end of the column three; X extends the other end to a closed four; O blocks again. Each forced O move lands exactly where X predicted, and X uses the time to add marks that build yet another line. A few moves later X finds a point that makes two fours at once — a double four — and wins.
Notice what made this work: every X four was made at a point that also built something else. Fours that only force a reply, without building, run out quickly and leave X with nothing.
Fours and the clock
In timed online games, fours have a practical side effect: your opponent must find the block, and even a simple forced reply costs a little time. Strong players do not rely on this, but it is worth knowing that a well-timed series of forcing fours can put real pressure on a defender who is short of time. It is not a substitute for a sound combination; it is a bonus when you have one.
Practice
Play the computer on the home board with this focus: every time you have a chance to make a four, pause and answer the three questions above before doing it. You will find that many of your fours were unnecessary, and that keeping them in reserve makes your later combinations much stronger. Replay your games from your account to find the fours that won and the ones that were wasted.
Frequently asked questions
What is an open four in gomoku?
Four marks in an unbroken row with both ends empty. It threatens five at two points and cannot be stopped.
What is the difference between a four and an open four?
A simple four has only one way to become five and can be blocked. An open four has two and cannot.
What is a broken four?
Four marks with a single gap, such as X X _ X X. Filling the gap makes five, so it must be blocked in the gap.
Should I always make a four when I can?
No. Make a four when the forced reply helps you or when it creates a second threat. Otherwise keep it in reserve.
Does an open four win under blocked-ends rules?
Yes. Completing five at one end leaves the other end empty, so the five is not blocked at both ends.
