Tier 3 · first asked for by Tough
Naked Quad
Four cells in a unit sharing four candidates between them lock those digits away.
A naked subset of size four. Four cells whose combined candidates are exactly four digits must consume all four, so those digits leave the rest of the unit.
Quads are rare and awkward to see, mostly because there are so many groups of four cells to consider. In practice a naked quad usually means the complementary hidden pair or triple was easier to spot from the other direction.
A worked example
A real position, with the pencil marks a solver would have at that point.
the pattern · what it rules out · 5 a candidate that goes
What it proves. R6C3, R6C4, R6C7, R6C8 in row 6 hold only 4/6/8/9 between them, so those digits leave the rest of the row.
Builds on Naked Triple.