At this age, your child will get more curious about understanding his body and body parts. He would have started identifying few body parts much earlier but at this age, his knowledge of his body would begin expanding.
What you need to know
4-5 years is the age when most kids start understanding their bodies better. Your kid might be able to match the pictures of parts of the body to his own body. He might even attempt at drawing some of them. Do not expect his drawing to be a masterpiece though. Sit with him and help him draw and appreciate his efforts in identifying and sketching his body parts.
Appreciate him and start talking to him about the basic anatomy of his body. While he starts exploring the parts of his body, if you find him looking at or getting curious about his private parts, grab the opportunity to talk to him.
This would also be an ideal age to teach him good touch and bad touch. When you normalise it and actually talk to him about it, you might help curb his curiosity and he might accept it with ease. This is the age when your kid would start understanding the difference and identify genders.