Fish
How it works
The same fish one size larger: four rows whose remaining cells for a digit all fall inside the same four columns. Four rows means four copies of the digit, one per column, so nothing else in those four columns can hold it. Rare in practice — by the time a puzzle needs a Jellyfish, the smaller fish have usually already broken it open.
Try it
No worked example for this one yet. It turns up rarely enough that no puzzle has needed it as its next step so far — one will appear here as soon as a generated puzzle offers it.