Utilize giant sets on occasion. A giant set is when you do at least four exercises for a single muscle group simultaneously without resting. Do one or two of these giant sets in order to shock a muscle into growing. For your smaller muscles, which include your shoulders, biceps, and trips, a single giant set is adequate in order to achieve a complete workout.
Make sure you are eating enough. Even if you are trying to lose weight while you build muscle, it is important that you are consuming sufficient calories. When your body is deprived of its fuel, it will be difficult to build muscle. An ideal diet for muscle gain is high in protein and low in fat and refined (processed) carbohydrates.
If your training regimen has reached four days weekly, then try to avoid having more than one pair of consecutive down days. Your body will build and recover better with the down days spread out. If your life and schedule make this impossible, do not lose any sleep over it though, as you are still working out four days a week.