Naked quad
Four cells in a unit hold only four digits between them.
A technique for Hard puzzles. Play a Hard puzzle
What it does
The pair and triple idea with four cells. If four cells of a row, column or box together hold only four different digits, they take those four digits between them, so no other cell of the unit can hold them.
How to spot it
With your notes written in, look for four cells in one unit whose candidates, put together, are only four digits. It’s rare, and easier to see in a unit with many empty cells.
What it lets you do
You remove those four digits from every other cell of the unit.
Try it
Find the Naked 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.