How can i lose 20 kgs in 4 months?

That's 5 kg/month: I don't think it is possible. Rapid weight loss tends to be temporary anyway. You need a long term strategy.

11 lbs / 5 KG per mo: is a steep lose. Consider losing more slowly. To lose weight insure that calories burned > calories consumed. Drink at least 8 - 8 oz glasses of water /day. Try for 8 hrs of sleep/night. Aim for at least 5 servings of vegetables & 4 of fruit per day. Use whole grains. Avoid sodas, alcohol & junk carbohydrates. Get in a good mix of aerobic & weight lifting exercises.