You should really Cast on about 24 sts because the edge sts curl under and aren't the same size as the ones in the middle. You want to try to match the st number, so measure over 16 sts in the middle. You can try going up another needle size or 2, but yes, if you're using thinner yarn than in the pattern, it affects the gauge. You might actually get 4 sts/inch, but may be too loose a gauge. What yarn are you using?
sue- knitting heretic