There are only two things to do in order to successfully answer the question of how to gain weight and muscle. Apply these two things to a 4 week program and you are going to come out stronger, tougher, and bigger.
1. Lifting: The backbone of a how to gain weight and muscle program is weight lifting. And performing the following exercises are the key to getting bigger. These exercises are known to put on mass quickly. They not only work the major mass muscles, but performing these exercises literally put your body in a growth zone that makes it easy and quick to gain weight and muscle. Any hardcore gym will verify that the fastest growth that members have experienced were through the practice of the following weight lifting exercises:
These are key on how to gain weight and muscle and so should be the main animals in your stable of weight training exercise. For instance you could perform a Monday, Wednesday, Friday workout schedule which would include one of these big dog exercises above for each workout day (for instance Squats on Monday, Dips on Wednesday, and Deads on Friday). You can then add other exercises into the mix, like bicep curls, chest exercises, etc. But these three should be your main focus as they are the absolute key on how to gain weight and muscle.
2. Eat. During your 4 week how to gain weight and muscle program, you must eat like you've never eaten before. Visualize each meal contributing to your gain weight and muscle goals, and if you miss one then you are just pushing yourself backwards and not forwards.
You should consume 3 strong meals a day and then 3 minor meals in between. Your breakfast, lunch, and dinner strong meals should all consist of at least 30-50 grams of protein from primarily a meat source. Just as the 3 above exercises are your backbones, these 3 meat meals are your eating backbones. In addition to your meat, you should eat large portions of carbohydrates and some fat in the same meals. Now in between your 3 strong meals you need to consume 3 minor meals, such as a protein shake or a couple of protein bars, or essentially some snack which is going to contain another minimum of 30-50 grams of protein, but you are allowed to have this in protein shake form or other easy, convenient protein forms. And the carbohydrate amount need not be as heavy. This is the ideal eating plan for how to gain weight and muscle.
With focus, dedication, desire and a solid plan you can indeed learn how to gain weight and muscle in 4 weeks. From that point you can gauge your progress, and if you want to continue to gain weight and muscle and get even larger, then you can take what you learned and set up another 4 week mass building program.