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.
このはこには、みかんが四つあります。
There are four oranges in this box.
小さい玉ねぎを四つ下さい。
Please give me four small onions.
今朝、クロワッサンを四つか五つ食べました。
This morning, I ate four or five croissants.