Quads
How it works
Four cells in the same row, column or box carry only four candidate digits between them, combined — not necessarily one each. Those four digits must fill those four cells between them, so every other cell in that unit can have them eliminated. The rarest of the naked subsets to actually need: by the time four cells narrow to four candidates, a smaller subset has usually already fired first.
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.