The same idea as Sudoku on a quarter of the grid. Four rows, four columns, four boxes, and the numbers one to four.
Quick enough to finish in a minute, which makes it a good place to learn the logic or to kill five minutes.
Every row, every column and every two by two box has to hold the numbers one to four exactly once.
Click a square and type or tap the number you want. The grid tells you straight away if it clashes with something.
With only four numbers, most squares resolve from one glance: if a row already has three of them, the fourth is forced.
There is a hint button if you want the next step explained, and the whole thing resets with New Game.
Yes. The grid is small enough to hold in your head, and the reasoning is exactly the same as the full size game.
Yes, a new one every day at each difficulty, and the previous games button goes back through earlier days.
Harder than it sounds. The tighter difficulties leave fewer givens, so you have to look at columns and boxes rather than just the row in front of you.
Yes, every puzzle is verified for a single solution before it goes up.
A four by four grid is where Sudoku logic is easiest to see. Everything that makes the full size puzzle work is already here, just with fewer places to hide.
It is also a good warm up. Players who find the nine by nine grid overwhelming usually find that a few of these make the bigger board feel less like a wall.
If you want a step up without the full jump, Killer Sudoku 4x4 adds cage totals to the same small grid.