A Hidden Quad occurs when four values appear as candidates in exactly four cells of a unit, and nowhere else in that unit. Those four cells must contain those four values, so all other candidates can be removed from those four cells.
This is one of the rarest basic techniques. After exhausting triples, check if four digits are confined to exactly four cells in any unit. The cells will typically have many other candidates obscuring the pattern.
If four values can only go in four cells, those cells are fully claimed. Any additional candidates in those cells are impossible.
Let's find a Hidden Quad in row 5. All cells are empty with many candidates. We need to find four digits that are confined to exactly four cells.