Hidden Quad
A Hidden Quad appears when four digits can only go in the same four cells inside one house — a row, column, or 3×3 box. Those four cells have to hold all four digits between them, so any other pencil-mark candidate sitting alongside them in those cells is dead weight and can be erased.
What is a Hidden Quad?
A Hidden Quad is the four-digit version of a Hidden Pair, and like its smaller cousins it's buried under extra candidates. The four cells that share the locked digits also carry other pencil marks, so you have to hunt for the digits that only appear in those four cells, not the cells that only have four candidates. Once found, the payoff mirrors a Hidden Pair: you strip the surplus notes from inside the quad cells, not from the rest of the house. It's the mirror image of a Naked Quad, which locks four cells to four digits and cleans the house instead.
How to spot it
Pick a house and, for each digit, count how many cells can still hold it. When four different digits each have exactly four candidate cells left, and those four cells are the same set, you've found a Hidden Quad. It's the hardest hidden subset to spot because you're juggling four digits and four cells at once, and the extra candidates in those cells camouflage the pattern. Scanning by digit rather than by cell is the reliable way in. Hidden Quads are extremely rare and tend to appear only in the hardest puzzles once all easier subsets have been exhausted.
The logic
Every house has to place each digit 1–9 exactly once. If four digits can only go in the same four cells in a house, those four cells must contain all four digits between them — one digit per cell — because none of the four appears anywhere else in the house. That fully commits the four cells: they'll hold exactly those four digits, in some order. Any other candidate note inside them is impossible, so it can be crossed off. The same reasoning with two digits gives a Hidden Pair, with three a Hidden Triplet, and with four it's the Hidden Quad.