Naked Quad
A Naked Quad is four cells in one house 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 house.
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 — the four cells need four different values, and only those four digits are available.
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 house 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 house.