The Jellyfish is the 4x4 variant of the X-Wing. A Jellyfish brings nothing new to the table. It behaves exactly like an X-Wing or Swordfish, except that it is based on 4x4 cells.
See the X-Wing and Swordfish pages to learn more about how (and why) the Jellyfish technique works.
You don't need to look for bigger fish
In standard Sudoku (9x9), you only need to look for X-Wings (2x2), Swordfish (3x3) and Jellyfish (4x4). For every fish that is 5x5 or bigger, there would always be a complementary fish, that is smaller in size, so you don't need to look for the bigger one.
Very common for Jellyfish
What is seemingly very common for Jellyfish is that there is a complementary Jellyfish. So for every Row-to-Column-Jellyfish there seems to be a Column-to-Row-Jellyfish.
That is a bit weird. Usually we would expect the complementary fish to be 5x5 in size. (9 rows/columns of the Sudoku grid minus 4 rows/columns of the Jellyfish). Strangely, I haven't found a single Jellyfish that doesn't have a complementary Jellyfish.
Here are some examples for complementary Jellyfish: