Babies put everything in their mouths to explore and understand the world around them.  Most kids grow out of it as mobility and function develop.  The best way to stop a biting habit is to address it as soon as it starts.  Even a baby who bites at the breast can be redirected and told "no biting".  Most times gentle warnings work but if your child continues to bite then investigate if they are biting to get attention, are they limited or overwhelmed sensory-wise, or they may need help finding others productive ways to express themselves. 

At 4 years old a habit is forming, so it's important to address it immediately.  Consistently say "no biting" sternly and use time out or loss of a favorite toy to reshape the behavior.  Also, they are old enough to understand that biting hurts others and that is unacceptable.  Reward good behavior and encourage using their mouth to talk and not bite.