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?

Kids are very observant. My daughter sometimes amazes me by talking about instances I never could have imagined would leave an impact on her memory. I think, here, there's nothing to feel worried about. If your wife's have got big breasts then he was justified in sketching what he sees. You should appreciate his observation rather than getting shocked. Now that you know how well observant your child is, encourage him, you never know, he becomes a fine painter one day.