Outs for a gutshot, an open-ender, a flush draw and a combo draw; ×2 gives your chance to hit on the next card.
4 → 8%, 8 → 16%, 9 → 18%, 15 → 30%. These are next-card numbers. On the turn (one card to come), compare them with the required equity (20-25-30-33-40) and use implied odds when short. On the flop (two cards to come), flush draws and open-enders usually call a bet of half the pot or less.