I'm smoking from past 4 years how much my lungs could be damaged? If I quit by now will they become healthy again?

Yes lungs do heal. How much did you smoke each day? What kind of cigarettes, filtered or unfiltered? Did any of your closer relatives develop emphysema (COPD) or lung cancer, with smoking? Do you have shortness of breath or trouble breathing after exertion (doe)? If not, you may do well if you quit cigarettes now, as the lungs, throat and mouth can heal in time. See your doc for regular checks in the future.
Yes they will. If you only smoked for 4 years, the damage is generally minimal (depends on 1 PPD or 4 ppd, filtered/unfiltered, how intensively you smoke). Once people stop, the damage begins to undo itself almost immediately. The pigment that tobacco produces in the lungs will not go away, but will lighten and lung function, if not already damaged, will be normal. Cancer risks go way down.
Quit NOW! The good news is you are still young and have not been "smoking for that long." nothing good comes from smoking, and the sooner you quit the better. Everyone's body is different, but at your age, stopping smoking can certainly help your lungs heal from any damage done and you have a good chance to have healthy lungs. Good luck, you need to quit now!