Go up to a 4mm then a 4.5 if you have them both, knit 2-3" with one size, then with the next, day don't have to be separate. Also, caston about 6 more sts than the gauge for 4". You want the swatch to be as large as possible because the end sts curl under and aren't the same size as the ones in the center. You measure over 4" in the middle.
sue- knitting heretic