Four Things
numeral
Any number with a つ attached is a word that means "NUMBER Things." In this case, it's four things.
Since this word consists of a kanji with hiragana attached, you can assume it uses the kun'yomi reading. The ん part of よん gets converted to a small っ though, so watch out for that.
四つのテスト
four tests
四つのタイプ
four types
四つのグループ
four groups
ミルクシェイク、四つ下さい。
Four milkshakes, please.
水に氷が四つ入っていた。
There were four ice cubes in water.
このアパート、クローゼットのかわりにオーブンが四つもある。
This apartment has four ovens in place of closets.