Google tells me that "#4 Worsted" yarn weight in America equals "Aran" weight in the UK and Europe. Is that helpful?
Also, cotton feels really nice, but it's not going to have the same kind of drape that wool or even acrylic might have, or what I would want in a cardigan. (But you're not me, so...) I've had good luck with superwash wool from Knitpicks. I've made a cardigan and a sweater with it. I spent about $100 US on a booty length cardigan amount of yarn (knit, not crochet, if it matters), so your price quote for floor length seems about comparable.
If you're in the UK, knitpicks.com will ship to you. Doesn't look like they go to Europe though. :(
I hope any of that helps some. Good luck on your cardigan adventure!