Hidden Quads
We can also extend Hidden Singles, Hidden Pairs and Hidden Triples to Hidden Quads. Although these are very rare,
they could be found in some of the higher difficulty boards. We need a collection of four cells within a house,
which have a group of four candidates and the candidates are seen only in these four cells. Again, it is not necessary
that all four candidates are contained in each cell, which is quite rare. It is enough that each cell contains some of
the group of candidates.
Hidden Quads - Example 1
In the following example, we look at row A and more specifically cells A1, A2, A3 and A7. The group of candidates {4, 5, 6, 8} is seen only in these four cells and nowhere else. Hence, we know that all four candidates will be the values in these cells in some order, but no other candidates can be in these cells. Hence We can remove the candidates in red.
