If i eat 4000 calories per day, as a sedentary person at the computer all day, will i gain weight?

Not a good idea at a: You're too young to be acting like an 85 year old inactive, sedentary person at your age. To gain weight involves muscle strength, exercising, eating a balanced diet with protein, good fats (monosaturated, omega 3's > 6'a. No one should eat 4000 calories a day unless you can burn it off by marathon or triathlon. If you haven't trained well, you could hurt yourself. See your doctor to discuss more.

Yes: For every 3500 more calories taken in than burned off you put on 1 pound. You need to exercise daily and decrease calorie intake.