Naked Quad

The pattern

Four cells in one unit whose candidate lists, combined, contain exactly four digits. Those four digits fill those four cells in some order and can be erased from the rest of the unit.

The individual cells can hold two, three or four candidates each. As with pairs and triples, only the size of the union matters.

When it is worth hunting for

Rarely, and that is an honest answer. A unit with four cells locked by a quad usually also contains an easier move — a hidden single, a pair, a pointing elimination — and finding that costs less effort.

The quad earns its place in one situation: a unit with five or six empty cells where four of them are clearly constrained. Then the elimination hits the one or two remaining cells hard, often solving them outright.

The shortcut worth knowing

In a unit with six empty cells, a naked quad among four of them is the same statement as a hidden pair among the other two. Whichever is easier to see, see that one — the eliminations you get are equivalent.

Why it is valid

Four digits, four cells, one appearance each per unit. Any outside cell taking one of the four digits would leave three digits to cover four cells, which cannot be done.