Best natural method to gain muscle mass in two weeks?

Protein, and weights. That’s the only good way to put on lasting and strong muscle. You might want to YouTube some videos of workouts that specifically target the muscle groups you want to work on.

The bench press is the single best exercise to add muscle mass to the upper body.

Doing a very hard thing over and over and over. Which is why lifting weights is popular. It’s hard to find someone putting in a railroad these days to practice on lifting and swinging a 50 pound hammer… or pushing something heavy uphill.
Chin-ups, push-ups and pushing door frames apart are the most “natural” ways to get buff (without a gym). Jack Lalaine used to advocate pushing against your own palms.
However, two weeks isn’t enough time to get buff. Depending on your age, you will need hard work for about 3 months before someone other than you can begin seeing a change and 6 months before any “noticable” change.
To obtain 4 Kg muscle you will need 3–6 months working very hard and eating right. I dont think you could even gain that much fat weight in 2 weeks if you tried. Sorry.

The best method is having more realistic expectations.

4 kg, or 8.8 pounds in two weeks is not only borderline impossible…. It’s ridiculously unhealthy.

The quickest you should be doing that is about a month, and honestly two would probably be the safe bet. When you factor in the down time you have to take in between working out to give your muscles time to regenerate and grow, your quest is likely an impossible one.

form is the most important to concern yourself with.. if you aren’t doing it right.. you might as well not do it at all.

I repeat, The bench press is the single best exercise to add muscle mass to the upper body.