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?

I am assuming this is the first time? He is still very young and learning. Do not fret too much over it. If you are too uncomfortable and he does this again, talk to him about it, gently. I hope you had a good laugh with your wife :D