Striped garter stitch problem

I’m working on a pattern that calls for working in garter stitch. Every 4 rows I am to switch colors. Unfortunately, when I knit the first row in the new color, it creates an unwanted stripe of the contrasting color in the last row of the main color. How do I stop this from happening?

Always change colors on the right or front side of your work. The back or wrong side will show the color change. Here’s a part of a baby sweater I made. You can see that it looks nice at the color change on the front side, but on the back or wrong side side you can see the problem.