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

You can begin brushing his teeth now. Get a brush with soft bristles and a non fluoride toothpaste. Make it a routine to make him brush his teeth. If you will not inculcate the habit of brushing teeth now, it is going to turn very difficult for you to make it a habit with him later. So, sooner, the better. http://raisingchildren.net.au/articles/dental_care_babies.html