Naked Quads
We can naturally extend the idea of Naked Singles, Naked Pairs and Naked Triples to Naked Quads. Although, these
are rare and slightly harder to spot, they can still be found in some boards and you should look
out for them. We are looking for a collection of four cells within a house, which have a group
of four candidates and nothing else. Again, it is not necessary that all four candidates are contained
in each cell - in fact this is very rare. It is enough that each cell contains some of the group of candidates.
Naked Quads - Example 1
Consider row B in the example below. The cells in dark blue form a Naked Quad with candidates {1, 3, 4, 7}. We know that cells B2, B4, B5 and B6 will contain these candidates, but we do not know in which order. In any case these candidates cannot be present in any of the other cells in row B. Hence we can eliminate all candidates in red.
