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.

Naked Quads made up of twos and threes like this are difficult to spot but if you’re sure they are present then it may crack a puzzle for you.

Are sets of five numbers possible, or six?  Theoretically yes, Quintets, Sextets, but it is highly unlikely they are easy to spot or any great addition to the armoury. 

There are two Naked Quads on this board. Can you spot them?  If you can find the first one the second will be revealed.   The circle means there is a candidate that can be eliminated there (among others) by the first Quad. 

Once that candidate has been removed another Quad will be formed and one more candidate exposed for removal.  Find that single candidate and you’ll have solved this exercise.  Answer on page X.