How does US worsted classification 4 compare with UK yarns in knitting?

See http://www.YarnStandards.com for how the categories of yarn weights compare in terms of relative gauges and needle sizes used to achieve those gauges.

Worsted weight (Category 4 - Medium) is the yarn commonly used to knit sweaters in the U.S. U.K. knitters often use a slightly thinner yarn called "DK" (double knitting), which falls into Category 3 - Light. Worsted weight yarn is typically knit at 20 sts/4 in or 10 cm on US 7/4.5mm needles, whereas DK weight yarn is typically knit at 22 sts/4 inches or 10 cm on US 5/3.75mm needles.

Some countries still use the old 2-ply, 4-ply, 6-ply, 8-ply classification to describe yarns, where 4-ply is fingering weight, 6-ply is sport weight, 8-ply is DK, and 10-ply is worsted weight. One problem with that system of classification is that it assumes a standard ply weight. There are many yarns these days (particularly in the Bulky and Super Bulky categories 5 and 6) that are single-ply yarns which knit at various gauges on various needle sizes.