This strategy is the inverse of naked quads and can be used to clarify remaining candidates in the grid, leading to other solving opportunities. It applies to rows, columns and subgrids but is easier to explain by discussing one of those at a time.
You're looking for a quadruple of candidate numbers used only in four different cells within the same column. They are harder to spot than naked quads as other candidates can also be present. As the quadruple of cells must contain all occurences of the four digits (but we don't know which order just yet), we can eliminate any other candidates in the quadrupled cells.
N.B. You should only use this strategy once all cells in the column (or row or subgrid) have been filled with candidates.
In the example, the numbers 2, 6, 7 and 8 form a hidden quad as they are only used in four cells of the column.
We can then eliminate any other candidates in those four cells, turning them into a naked quad: