Look at box 9 of the puzzle. Four cells are highlighted in blue at G7, G8, G9 and H7. Collectively, they contain only four candidates,
namely 2, 3, 7 and 8. Although we can't say at this stage which cell will be solved by each number, at least we can say that none
of the other cells in box 9 can have 2, 3, 7 or 8 as a solution. So, referring to the candidates highlighted by a magenta background,
we can say that I9 cannot have a solution of 2, 7 or 8. These three candidates may therefore be removed.
Naked quads may also appear in rows and columns.