A Naked Quad occurs when four cells in a unit collectively contain exactly four candidates. Each cell may have two, three, or four of those values, but the combined set is exactly four. Those four values must occupy those four cells, so they can be eliminated from all other cells in the unit.
After exhausting triples, look for four cells in a unit whose combined candidate set has exactly four values. This is relatively rare but powerful.
If four cells can only hold values from a set of four digits, those four digits are locked into those four cells. No other cell in the unit can contain any of them.
Let's find a Naked Quad in column 1. Several cells have candidates and we need to identify four cells whose combined candidates form exactly four distinct digits.