A naked quad involves four cells in a house that contain only candidates from a set of four digits.
Four cells collectively contain only four different candidates.
Like triples, not every cell needs all four candidates.
Various combinations possible: {1,2,3,4}, {1,2}, {3,4}, {1,3,4}, etc.
These four digits are locked to these four cells.
Eliminate these four candidates from all other cells in the house.
Naked quads are rare and hard to spot due to many subset combinations.
Cells with {1,3}, {3,6}, {1,6,9}, {3,9} form a quad with digits {1,3,6,9}.