I only eat 1500 calories a day, and I work out for about an hour four times a week but I'm stuck at 140 lbs. Why can't I lose weight?

I assume you have lost weight and now you can't lose anymore. This is what is known as hitting a plateau. You will stay there for awhile regardless of what you do. This is your body's way of adjusting. However after you become discouraged one day you will start to lose again so don't give up.