A Naked Quad is rarer, especially in its full form but still useful if they can be spotted. The same logic from Naked Triples applies to combinations of numbers. This list is long but it starts
(1234) (1234) (1234) (1234)
And can be a tortuous foursome like this
(12) (23) (34) (14)
Here’s an example of a Quad in a row.
We have 1/3/4/8 in four cells, two cells containing all 4, two just 3.
Those numbers can’t be anywhere else in the row so they can be removed from other cells in the row. 1 and 8 from H1 for example, 8 in H2, 1 in H3 and 8 in H8.