Hidden Quad
A Hidden Quad occurs when four digits are confined to a total of four cells in one row, column, or box. The four candidate distributions need not be identical. Other candidates can be removed from those four cells.
What is a Hidden Quad?
For four selected digits, take the union of all their remaining positions in one sector. If that union contains exactly four cells, those cells must hold the four digits between them, so no other candidates can remain in them.
How to spot it
Track the available positions for each digit in one sector. Look for four digits whose position union is four cells, even when some digits appear in only two or three of them. Do not require each digit to appear in all four cells.
The logic
All four selected digits must occur in the sector, and only the selected four cells can contain them. Those cells are therefore reserved for the four digits. Remove every other candidate from the selected cells.