Hidden Quadruple in Sudoku

Hidden Quadruple

Hidden Quadruples—an occurrence where exactly four unique digits are found solely in four cells within one house—are fairly uncommon and usually complicated to spot without using pencil markings.

Example1:

In example one, it illustrate such scenario in Block 8: the four additional cells R7C5, R7C6, R8C5 and R8C6 are also included. Over these four positions, only candidates 2, 4, 5, and 8 are available which clearly suggests a Hidden Quadruple.

Example2:

In the second example, the subset is noted with 2, 4, 5 and 9 in Row 9, specifically in R1C9, R2C9, R3C9 and R8C9. As these four digits are present only in those four cells, they are free to have all pencil marks in any position of the house.

How to find hidden subset

Detecting Hidden Subsets, in particular Pairs, is greatly simplified with the use of pencil marks. For instance, in this case it is clear that cell R3C4 and R3C6 contain the Hidden Pair of 4 and 8. Because none of the other cells in Row 3 contain these digits, both cells must contain the rest of the digits.

Identifying larger Hidden Subsets, such as Triples or Quadruples, often demands acute short-term memory because the patterns are more intricate and less visually apparent. Due to this, many players prefer to use Naked Subsets after all candidate marks have been filled in, although ultimately it is a matter of personal preference.