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

As early as the first tooth appeared. It's also to get them into the habit before it's too late. You can also make it more fun by buying your lo those funny finger brushes or cartoon toothbrushes. You may not use toothpaste at this time as the brushing motion would be enough. I assume you do not give you lo sweets yet so this should be fine. Have fun! :-)