Yes worsted weight usually knits to about 20 stitches to 10 cm on 4 mm knitting needles and DK is 22 stitches to 10 cm on 4 mm knitting needles (and sometimes 24 stitches depending on the manufacturer). I'd always suggest making a tension swatch and checking it against the pattern before you begin knitting if you're not sure.
Some of the UK yarn designers are bringing in yarns which although they're DK knit to a Worsted tension and it's always worth checking this before substituting yarn for a pattern knitted for US worsted weight. Rowan tweed and some of Debbie Bliss's yarns are two which come to mind. You may like to check out
www.angelyarns.co.uk and
www.getknitted.com if you are in the UK, also
www.laughinghens.com I believe are good though I haven't personally used their services. They're always being mentioned in the UK knitting groups!