4-cell Kakuro run summing to 27

3 digit sets sum to 27 across 4 cells. The run on its own is permissive; its constraints become useful when intersected with the perpendicular runs at each of its cells.

Locked digits: 9. This digit appears in every valid combination, so it must occupy one cell of the run; useful when a perpendicular run rules it out of a candidate cell.

Forbidden digits: 1, 2. None of these digits appears in any valid combination, so you can rule them out of the run's cells immediately.