Right, do some more increase rows, you'll probably need more stitches on the body too because you have a smaller gauge. Do this until the back stitches measure about 12½". That's how large the sweater is supposed to measure with a 4 st per inch gauge. That will also increase the sleeves, and then on the last row before dividing the sleeve and body stitches, do the inc at the beginning and end of the row as it says in the pattern. That should give you about 40 sts or so on the sleeves.
sue- knitting heretic