Work the kp rib over 4 sts, bind off 2 sts, then continue the rib pattern to the end of the row. Next row, work in the rib pattern till you get to where the sts were bound off and cast on 2, then work in the rib on the last 4 sts.
It doesn't say 'k1p1k1p1, bo 2 sts, etc' because you might be on a row that begins with a purl, so you'd begin the row p1k1p1k1. This way you can put the buttonhole on either side for a girl or boy sweater.
sue- knitting heretic