Jellyfish (Size-4 Fish)

A Jellyfish is a size-$4$ fish.

When a candidate digit $d$ in four rows or columns (the base set) falls only in four columns or rows (the cover set), then no matter how $d$ is placed, each of the four cover-set houses must contain exactly one digit $d$. Therefore, candidate digit $d$ can be eliminated from all other cells in the cover set.

For example, in the board below, take four rows as the base set. Look at candidate digit $5$ in row $1$, row $4$, row $8$, and row $9$: it appears only in column $1$, column $2$, column $5$, and column $8$. So regardless of how these four $5$s are arranged within row $1$, row $4$, row $8$, and row $9$, column $1$, column $2$, column $5$, and column $8$ must each contain one digit $5$. Therefore, in those four columns, candidate digit $5$ can be eliminated from every cell except the intersections of the base set and the cover set.

Note that in row $1$, row $4$, and row $8$, candidate digit $5$ does not appear exactly four times, but that does not prevent them from participating in a size-$4$ fish. It is enough that all occurrences of candidate digit $5$ in the entire base set lie within the cover set.

For another example, in the board below, take four columns as the base set. Look at candidate digit $6$ in column $1$, column $5$, column $7$, and column $9$: it appears only in row $1$, row $4$, row $7$, and row $9$. So regardless of how these four $6$s are arranged within column $1$, column $5$, column $7$, and column $9$, row $1$, row $4$, row $7$, and row $9$ must each contain one digit $6$. Therefore, in those four rows, candidate digit $6$ can be eliminated from every cell except the intersections of the base set and the cover set.