Tier 3 · first asked for by Tough
Hidden Quad
Four digits confined to the same four cells clear everything else out of them.
Four digits in a unit that can only go in four cells between them. Those cells hold exactly those digits, and every other candidate in them can be removed.
The hardest of the subsets to see by eye, and the least often needed. When it appears, it is usually in a unit with only a handful of cells left, which is what makes the counting tractable.
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. In column 8, 1/2/3/7 can only live in R1C8, R4C8, R7C8, R9C8, so those cells hold nothing else.
Builds on Hidden Triple.