It's generally a good idea to CO more sts than you need to make the 10cm/4", so 30 sts would be good to do and don't include the edge sts when you measure. They're a different size than the middle sts and curl under. So you should get 24 sts over 10cm across and 36 rows in 10 cm up and down. Hardly anyone gets both row and st gauge right, but the sts are more important as you can knit less or more rows to get the length which is usually given as a measurement anyway.
sue- knitting heretic