Four Things
numeral
Any number with a つ attached is a word that means "NUMBER Things." In this case, it's four things.
よっつ
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.
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.