If you bind off all the stitches at once, you'll have a straight line of bound of edge. By doing it in steps, you'll have a more gradual decrease at the neckline.
Make sure you bind of only at the beginning of the rows--you can't bind off at the end of a row. So you'll bind of the first 4 and work to the end. At the beginning of the next row you'll bind off four and work to the end. Do the same with the 2 stitch bind offs and then do it twice each side for the one stitch bind off.