Next, if you want to build muscle in 4 weeks, you should make sure you're eating enough calories. A good way to determine how much you should be eating is to figure out the number of calories you eat on a normal day. After you get this number, you should add 500 to it. So if you find that you were eating 2500 calories per day, you should now be eating 3000 calories per day. This will ensure that your muscles are getting enough calories to repair themselves and grow.