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.
In general, dk and worsted mean much the same thing. Dk is used in the UK and worsted in the US. However, there is a lot of variation between brands. The best idea is to check the length and weight information on the ball bands. I do find that US worsted weight patterns make up well in dk, but I always have lots left over! http://www.theknittingsite.com/knitting-conversion-tables/
There are two basic techniques for spinning wool: woolen and worsted. In woolen spinning, the fibers are oriented perpendicular to the yarn being made, while in worsted spinning the fibers are oriented parallel to the yarn being made. Woolen fibers are carded, creating a fuzzier, softer yarn. Worsted fibers are combed creating a smoother, harder yarn. Woolen yarns are generally preferred by knitters while worsted yarns are generally preferred by weavers.
Tessa Lorant has written: 'Knitted shawls & wraps' -- subject(s): Knitting, Patterns, Scarves, Shawls, Stoles (Clothing) 'Yarns for textile crafts' -- subject(s): Textile crafts, Yarn 'Hand and machine knitting' -- subject(s): Knitting, Machine knitting 'The Batsford book of handand machine knitting' -- subject(s): Knitting 'Yarns for the knitter' -- subject(s): Knitting, Yarn 'Knitted quilts & flounces' -- subject(s): Knitting, Ornamental Borders, Patterns, Quilts
DK stands for Double Knitting. You can find a comparison chart for yarns at Craft Yarn Council's web site. DK yarn (#3) is a little thinner than worsted weight (#4) which may or may not be important, depending on what you're making. When in doubt, make a gauge swatch with the yarn and hook you plan to use for your project.
You can get a rough idea by comparing it with a known sample, being careful not to stretch the yarns. You can weigh the skein and measure its length, and see how that compares with the numbers for the known yarn. You can use a double knitting pattern and make up a swatch to see if the size is approximately right. Even yarns with the same designation can vary quite widely.