How do I prevent frequent cold and cough for my 4 year old child?

At this age kids immune system is building up. So it is vulnerable. You should watch out for not exposing the child to the places where there is dust or to crowded places. I know it is difficult to make a small kid wear a mask when you step out, but if you think there's too much of pollution, you too wear a mask so that he doesn't mind wearing one himself. Wear a mask especially when you are going out in an auto or in a scooter. I am sorry to say but our thought process has become such that we also do not want to do extra things (like wearing a mask or using sanitiser every now and then) because we know that in our country pollution levels are so high and if not today then tomorrow the kid will have to exposed to such environment, so better not use to many of shields to protect and get him immuned to the polluted surroundings. But till the time, his system gets tough, you can try home remedies. I have personally used sitopaladi mixed with honey for my daughter's recurrent cough when I was fed up of giving her antibiotics every other month, and did wonders for her. Try it, and hope it works for your baby as well. http://www.thehealthsite.com/diseases-conditions/natural-remedies-for-frequent-cold-and-cough/ https://www.ayurtimes.com/sitopaladi-churna/