A hidden quad is four digits that, within one house, fit only into the same four cells. Those cells must hold exactly those four digits between them, so every other pencil mark inside them is noise and can be struck.
To spot one, sweep the house digit by digit and note each digit’s possible cells. You are looking for four digits whose lists all stay inside one set of four cells. The lists are usually ragged — one digit fits two of the cells, another three — and none of it needs to be tidy: only the union matters. Hidden quads are famously well camouflaged, which is why they sit this high on the ladder.
It works because the house must place all four digits, and only those four cells can take any of them. Four digits into four cells fills them completely — a fifth candidate surviving in one of the cells would leave the house trying to fit four digits into three cells.
On the board
Sweep row 4 on this board. The digit 2 fits only at R4C8 and R4C9; the 3 only at R4C7 and R4C9; the 5 only at R4C3 and R4C7; the 6 only at R4C3, R4C7, and R4C8. Four digits, and every home falls inside the same four cells — R4C3, R4C7, R4C8, and R4C9. Those cells must hold the 2, 3, 5, and 6 among them, so the clutter goes: R4C3 drops 4, 8, and 9 to become {5,6}, R4C7 drops its 4 and 8, and R4C8 sheds 1, 4, 8, and 9 to come out a clean {2,6}.
This is the exact board the app's interactive Hidden Quad demo uses — step 15 of its 65-step walkthrough, drawn here with its live pencil marks.
