Margaret

[GrammaM]

Size crochet hook works best on medium 4 yarn

[greyhoundgrandma]

If you are following a pattern it will tell you what size to use. The label on the yarn will give a suggested size. But even then it depends on your tension and you may have to use a different size to get the proper gauge (again the pattern usually gives a gauge).

It also depends on what you are making. Amigurumis (and often baskets) have tighter stitches to keep the stuffing in. A larger hook for say a blanket gives you a softer fabric.

Typically # 4 yarn uses an H, I, or J.

Hope that helped instead of totally confusing you.

[Granny Square]

^everything Greyhoundgrandma said.  Here are the US standards for yarn weights, it gives an average gauge range of stitches per inch and typically applicable hooks for each yarn weight. It gives the hook size in mm, but look at the menu on the right side of the page - another page gives you the mm to US 'size' cross reference, under 'hooks and needles'.

Underlining what Greyhoundgrandma said about everyone having different stitch tensions (we are all human and different), so what works for me might be completely different for you.  For many patterns I hit the designer's gauge spot on with the first try, but on some occasions I've had to move several hook sizes one way or another to match their gauge.