Guage help for buttony sweater

I impulsed bought 8 balls of Paton Rumor, so I was figuring I could make it into the Buttony Sweater. But I’m having a bit of a gauge issue. While she doesn’t say it on the pattern, on it was said her gauge is around 15 sts and 20 rows = 4 inches. With my US 9s I’m getting 12 sts and 18 rows. I’m confused on what to do, or how I should alter the pattern!

P.S. - This is my first full size sweater!

Try going down a size to 8s/5mm, though that seems kinda small for chunky yarn. To adjust, CO a different number of stitches, a number that can be divided by 4 because of the 2x2 ribbing on the front.