Hidden Quad
Four heights only fit in the same four cells of a row or column — strip those cells down to just those four.
The solver does not attempt this technique, so Smart Hint will never suggest it. It is documented because you may still want to spot it yourself.
How it works
- 1A 6x6 board, so heights run 1 to 6. Track the top four — 3, 4, 5 and 6 — across this row.
- 2None of them fits in the first two cells, which are down to {1, 2}.
- 3That confines four heights to four cells, so those cells hold exactly those four between them.
- 4Strike the leftover 1s and 2s. As with every hidden subset, no cell is settled yet — the row is just four eliminations tidier.
- 5This shape needs a board bigger than plain Skyscrapers' 5x5 to say anything at all, which is why it's rare until you're deep into Skyscrapers+. Even here the two cells it leaves alone are a Naked Pair on {1, 2} that clears the same marks sooner: a Hidden Quad only leads in a line of nine blanks, which is Skyscrapers+ at its largest.
- Height ruled out
- Relationship — The arrow shows which cells support the deduction.
- Conclusion — The final arrow lands beside the proved or ruled-out height.