Jellyfish
A Jellyfish is the four-line version of an X-Wing. Pick a digit and look for four rows (or columns) where that digit's candidates are confined to the same four columns (or rows). The sixteen intersection cells form the pattern, and because each of the four lines must contain the digit once, the digit has to occupy four of those cells — one per line. That locks it out of every other cell in the four shared lines.
What is a Jellyfish?
A Jellyfish is a single-digit fish pattern that spans four rows and four columns at the same time. The setup is: in four different rows, a particular digit can only go in cells that fall within the same four columns. The sixteen cells where those rows and columns intersect form a 4×4 grid — the "jellyfish". Because each of the four rows has to place the digit once, and the only available spots are inside that grid, the digit must land on four cells — one per row, and necessarily one per column (otherwise a column would get two copies, which is impossible). So each of the four columns receives exactly one digit from the Jellyfish, and every other candidate for that digit in those four columns can be erased. The same logic works with rows and columns swapped.
How to spot it
Jellyfish are the hardest fish to find because you're tracking four lines at once. The reliable way in is the same digit-first scan used for X-Wings and Swordfish: pick a candidate digit and, for each column, note which rows it can land in. If four columns each restrict that digit to at most four rows, and those row-sets all draw from the same four rows, you've found a Jellyfish. The cells don't have to be filled in every intersection — what matters is that the digit's candidates in those columns stay within the four shared rows. The same scan works row-first. Jellyfish are extremely rare and tend to appear only in the hardest puzzles.
The logic
Every row and every column places each digit 1–9 exactly once. If a digit is confined to four rows in each of four columns, and those rows are the same four across all four columns, then the four columns must place the digit in four of the sixteen intersection cells — one per column. The four placed digits necessarily fall in four different rows (otherwise some row would get two copies, which is impossible). So each of the four rows receives exactly one digit from the Jellyfish, and no other cell in those rows can hold it. The same argument with two rows gives an X-Wing, with three a Swordfish, and with four it's the Jellyfish.