4 Weight Yarn
CYC 4 weight is also called Medium, Worsted, Afghan, or Aran. This weight of yarn is the most commonly used yarn weight, it is easy to work with especially beginners and the best weight for afghans. Crochet hook sizes 5.5 mm to 6.5 mm or sizes I-9, J-10, and K-11. Knitting needle sizes 4.5 mm to 5.5 mm or sizes 7, 8, and 9.
Source: Craft Yarn Council of America's www.YarnStandards.com