A 4x4 Sudoku has sixteen cells, four rows, four columns and four boxes of 2x2. Every row, every column and every box holds the numbers 1 to 4, each exactly once. That is the whole rule, and it takes about ten seconds to state.
The interesting part is that a 4x4 grid can be solved completely by one technique, and it is not the technique that adults use on a 9x9. Knowing which one it is makes the difference between a child who finishes every puzzle in the book and a child who stalls on the third page.
The one question that solves the whole grid
Look at any row, column or box that already has three of its four cells filled. Ask: which number is missing?
There are only four numbers. Three of them are in front of you. The fourth goes in the empty cell. There is nothing to work out — you are reading, not reasoning.
Then look again. Filling that cell has just completed part of another row, another column and another box, so somewhere on the grid a new unit now has three of four. Find it. Repeat.
That is the entire method. A well-made 4x4 puzzle can be solved from start to finish this way, without ever writing a note, and without a single moment of genuine uncertainty.
Why this is not how you solve a 9x9
This is the part worth understanding, because it explains what the 4x4 stage is actually for.
On a 9x9 grid, "which number is missing from this row?" is almost never useful. A row with three empty cells has three missing numbers and no way to tell which goes where. So adults solve 9x9 grids by asking the question the other way round: where does this number go? You take a 7, look at a box, and eliminate the cells that are blocked by a 7 already sitting in their row or column. That is cross-hatching, and it is the workhorse of ordinary Sudoku.
On a 4x4 you never need it. The units are so small that the "what is missing" question always finds a cell to fill, and it always finds it faster.
So the two grid sizes really are answering different questions:
| Grid | The question that works | What you look at |
|---|---|---|
| 4x4 | What is missing from this line? | one row, column or box |
| 9x9 | Where does this number go? | one number across a whole box |
A child who has done two hundred 4x4 puzzles has become fluent at a question that stops paying off later. That is fine — it is the right question for the grid in front of them — but it explains a stall that surprises a lot of parents. A confident 4x4 solver can hit a 9x9 and freeze, not because the grid is bigger, but because they are asking a question the grid will not answer.
Working through one
Say the top-left 2x2 box holds 3 and 1 on the top and an empty cell beside a 4 underneath. The box has 3, 1 and 4, so the missing number is 2. Write it.
That 2 has just completed the bottom row of the box, and it also sits in the grid's second row and first column. Check all three. If the second row now has three of four, fill its last cell. If not, look at the first column.
Then move on. On a sixteen-cell grid you will typically find the next available cell within a few seconds of scanning, and the puzzle unravels in a chain — each answer creating the next one.
When nothing is obvious
Occasionally a 4x4 will present a moment where no unit has three of four. Most children read this as being stuck. It usually is not.
Two things to try, in order:
Count from the other direction. Instead of looking for a unit with three filled cells, pick a number — say 3 — and check whether it already appears in three of the four rows. If so, it has to be in the remaining row, and usually only one cell in that row is free of a 3 in its column and box. This is cross-hatching in miniature, and a 4x4 is a gentle place to meet it for the first time.
Check whether a mistake happened. On a genuine puzzle from a properly generated book, a 4x4 will not deadlock. If a grid truly cannot progress, a number is in the wrong cell. Look for a duplicate: the same number twice in a row, a column or a box. That is almost always what has gone wrong.
What not to do is guess. A 4x4 has few enough cells that guessing sometimes works, which makes it worse rather than better — it teaches a habit that fails completely at larger sizes. If a child is truly stuck, help them find the line to look at rather than the number to write.
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Sudoku for Kids 4-6
Colorful and fun sudoku-style puzzles for young learners. Build early logic skills with age-appropriate challenges.
Learn more →Does it need pencil marks?
No, and it is better without them.
Pencil marks — writing small candidate numbers into empty cells — are a 9x9 technique, and on a grid that size they are close to essential at harder levels. On a 4x4 they are slower than simply looking. Introducing them early also robs the child of the thing 4x4 is best at teaching: holding a small amount of information in your head and acting on it.
If a child wants to write candidates on a 4x4, that is usually a signal they are ready for a bigger grid rather than a sign they need the technique.
Where 4x4 sits in the progression
4x4 is the entry point, not a warm-up act. A child who can solve one reliably has learned the actual rule of Sudoku and the habit of checking three regions before writing — and both transfer completely.
What comes next is 6x6, where the boxes stop being square, and that transition has its own quiet difficulty worth knowing about in advance: 4x4 vs 6x6 Sudoku for kids covers it. If you are earlier than that and wondering whether your child is old enough at all, what age can kids start sudoku answers that question, and how to teach sudoku to kids covers running the first few sessions.
Among our books, Sudoku for Kids 4-6 spends the most time on 4x4 grids and is the right starting point if this stage has not happened yet. Sudoku Puzzle Book for Kids includes 120 4x4 puzzles inside a wider mix, and Sudoku for Kids Ages 6-8 starts at 4x4 and climbs through six levels. All ten are compared side by side on the sudoku books page.
The short version
Ask what is missing from a line. Fill it. That answer completes another line, so ask again. Repeat until the grid is full.
Do not ask where a number goes — that is the 9x9 question, and on a 4x4 it is extra work for the same answer. Save it. It becomes the main event soon enough, and the child who arrives at it having already made "check the row, the column and the box" automatic will pick it up far faster than one who started there.
