You have opened, your partner has answered One in a major and you have four-card support.
You need to show your fit and your zone of points.
Let’s say that you have opened 1♣ and that he has answered 1♠.
1♣ – 1♠
2♠: fit in Spades (at least 3 cards) and 12-14.
3♠: fit in Spades, 15-17 and a singleton (otherwise you would have opened 1NT)
4♠: fit in Spades, 18-19 without a singleton
4♦: fit in Spades, 18-19, singleton Diamond (Splinter)
4♥: fit in Spades, 18-19, singleton Heart (Splinter)