Today my 4 year old son drew a sketch of our family where he has shown his mother's breasts very elaborately. I was shocked to see his observation at such a young age. Should I do something or ignore?

By elaborate, do you mean an accurate rendering of breasts (with nipples) or just the outline? At his age, I think there is absolutely nothing to worry about. Children do not know what is taboo, "wrong" or indecent at that age and personally, I think it's sad that they have to grow out of that innocence. Your son is just drawing what he has innocently observed. Unless ingrained in them that it's somehow bad or wrong to know what the human body looks like, children will just draw whatever they see. My opinion is that you just ignore it for now. He doesn't mean anything negative or indecent. If you're really uncomfortable, then gently suggest that he should be less elaborate with his pictures. You can show him examples of pictures from his story books.