When a vocab is alone like this with no okurigana (hiragana attached to the kanji) it usually uses its kun'yomi reading. Numbers are an exception, though, and use the on'yomi reading. However, four and seven actually use both. We're going to go with the kun'yomi reading here, and since you haven't learned this reading yet, here's a mnemonic to help you:

You have to count to four. But you're tired. One... two... three... *YAWN* ... four. There, you did it. But *yawn* now you can't stop yawning. You're so, so sleepy. Four is too high a number to count without falling asleep.

By the way! Both the reading よん and し are pretty common, but because the on'yomi reading し sounds just like the word for "death," it isn't used as much. Generally when you're just counting (for example, "1, 2, 3, 4..."), you will use the reading し. But when you are counting things or people, you'll use よん. There are exceptions to this, but you'll learn them as they come up.