Hidden Quadruple

A hidden quadruple occurs when, in a house (row, column, or box), four digits can appear in only four cells, while those four cells still contain other candidates. In that case, all other candidates can be eliminated from those four cells. No matter how the digits are arranged, they must occupy those four cells.

Hidden Quadruple is a size-$4$ subset.

A hidden quadruple can appear in a row. For example, in row $5$ below, digits $3$, $7$, $8$, and $9$ can appear only in $R5C4$, $R5C5$, $R5C6$, and $R5C9$, so all other candidates can be eliminated from those four cells.

A hidden quadruple can also appear in a column. For example, in column $3$ below, digits $2$, $3$, $8$, and $9$ can appear only in $R4C3$, $R5C3$, $R6C3$, and $R9C3$, so all other candidates can be eliminated from those four cells.

A hidden quadruple can also appear in a box. For example, in box $3$ below, digits $1$, $3$, $7$, and $8$ can appear only in $R1C7$, $R1C9$, $R2C7$, and $R2C9$, so all other candidates can be eliminated from those four cells.