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

See 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.