You hold a hard 9 (no ace counted as 11). The dealer shows a 4. Here is the mathematically correct basic-strategy play — and why.
| Double ← | +0.1820 |
| Hit | +0.1290 |
| Stand | -0.2111 |
| You bust on the next card | 0.0% |
| Dealer 4 busts | 39.4% |
Double is worth 0.0530 more per unit than Hit. On a $10 bet that is $0.53 handed over every time you get it wrong. Across the 340 situations in this chart it ranks 102nd by expected value — better than 238 of them, worse than 101. Expected value is per unit staked, computed for multi-deck, dealer stands on soft 17, double after split allowed. The model and the code are published as data.
Hard 9 is a premium doubling hand. One card is very likely to give you a strong total, and against a 4 (which busts 40% of the time) you want maximum money on the table while you have the edge. Doubling wins you more in exactly the spots where you're a favourite.
If doubling isn't allowed (e.g. after hitting), simply hit instead — the extra card logic stays the same, you just can't add the extra bet.