My son has got 4 teeth now. When do you need to start brushing their teeth?

You can definitely start now. Get him a baby toothbrush with soft bristles and make it a habit to clean his teeth everyday, if possible, twice. If you start this habit early, you won't have to deal with them brushing teeth in the morning when they turn 2 or 3. Kids are known to throw tantrums when it comes to brushing teeth :)