|A hidden quad occurs when four numbers (a,b,c, and d)
are confined to four cells in a house.
Also one or more of these cells must contain other candidates other than a,b,c, and d
(otherwise it would be a naked quad).
In column 3, candidates 1267 are confined to the four yellow cells. Any other candidates in
those cells can be eliminated.
Column 3 also has naked quads (3458). This will perform the same eliminations as the hidden quad
(click here to see it)