If you are at a 200-300 calorie deficit in an attempt to lose weight then 30 minutes of cardiovascular exercise 4 times a week is enough to burn fat, but you will also be losing a little bit of muscle mass (you can limit the amount of muscle mass lost by increasing your protein intake, but you'll still be losing some muscle). If you are trying to lose weight that's a good thing. However, if you consume 200-300 hundred EXTRA calories a day in an effort to gain muscle mass then 30 minutes of weight lifting (or body weight exercises) 4 times a week is enough for you to gain some muscle mass, but you will also be gaining a small amount of fat.
I'm short, exercising for 30 minutes 4 times a week is very beneficial. It's enough to lose fat in a healthy way OR gain some healthy muscle mass depending on the type of exercises you are doing and your caloric intake. However, it's physiologically impossible to simultaneously build muscle and lose fat.