Naked Quad
A Naked Quad is four cells in one sector whose combined candidate set contains exactly four distinct digits. Those cells must hold those digits, so the four digits can be removed from every other cell in that sector.
What is a Naked Quad?
Choose four cells and take the union of their candidates. A valid Naked Quad has exactly four distinct digits in that union. Fewer would leave four cells without enough values in a consistent candidate state; more would not lock the set.
How to spot it
Look for four cells in one row, column, or box whose combined candidates are limited to the same four-digit union. The cells need not each contain all four digits. Then check other cells in that sector for any of those four candidates.
The logic
The four selected cells require four different values, all drawn from a four-digit union. They therefore use all four digits between them. Those digits are reserved inside the quad and can be removed from other cells in the sector; the deduction does not claim the digits lacked outside positions beforehand.