Hidden quad
Four digits fit in only the same four cells of a unit.
A technique for Hard puzzles. Play a Hard puzzle
What it does
The hidden pair and triple idea with four digits. If four digits can only go in the same four cells of a row, column or box, those cells hold those digits, and nothing else can go there.
How to spot it
It’s very rare, and almost only shows up in a row, column or box with no digits placed yet. In such a unit, check where each digit can go; when four of them share the same four cells and no others, you have a hidden quad.
What it lets you do
You remove every other candidate from the four cells, leaving only the four digits.
Try it
Find the Hidden quad on this board. Pick the cells it clears, mark the candidates it removes, then check.
These are the notes after the easier steps: some candidates are already removed, as they would be in your game.
Practise it
The best way to learn a technique is to meet it in a real game. Practice puzzles are sure to need it, at Hard level; any Hard puzzle may need it too.