Naked Quad

Hard Used by Smart Hint

Four cells in a row or column whose candidates combine to exactly four heights — eliminate those heights elsewhere in the line.

Cells that establish the pattern
Where the conclusion lands
Height ruled out

How it works

Four cells whose candidates between them cover only four heights: 1, 2, 3 and 4.
Four cells, four heights — all four are consumed inside that group, whatever order they end up in.
Strike them from the row's remaining cell, which leaves it a 5.
Quads are the last naked subset worth hunting in a five-line: a five-cell version would have nothing left to eliminate from.