A worsted weight yarn should be 4.5 to 5 stitches to 1 inch. What Plymouth yarn are you using? Is it Encore? Try knitting a swatch on size 9 or 10 needles to increase your gauge. However, this might result in a looser fabric. Make sure you like the feeling of it before knitting a whole sweater.
If the pattern asks for a gauge of 4 sts to 1 inch, that's a heavy worsted or aran weight yarn.