I always go up a size in jumpers and shirts and like Jennifer said, you'll get a couple of years out of them. Kids don't grow as rapidly as they do between ages newborn and 4 until they are teenagers.

DD is very tiny, her size 6 tops from prep still fit her, she's in grade 2 now. DS started school in a size 8 and they fitted him until grade 2 or 3 as well.