Tier 4 · first asked for by Diabolical
Jellyfish
The same fish pattern again, at size four.
Also called Fish of size 4.
Four rows in which a digit is confined to four columns between them. The digit fills one cell per row and one per column, so it leaves those four columns everywhere else.
Size four is the practical limit: a fish of size five in a nine-cell grid is equivalent to a smaller fish in the complementary lines, so there is never a reason to look for one.
A worked example
A real position, with the pencil marks a solver would have at that point.
the pattern · what it rules out · 5 a candidate that goes
What it proves. Jellyfish on 8: in column 2, column 3, column 4, column 5 the digit is confined to row 1, row 7, row 3, row 9, so it leaves those rows elsewhere (R3C8, R3C9).
Builds on Swordfish.