Remember when your kiddo was 2 years old and you used to teach him numbers like 2 apples, 3 dogs, 4 grapes, etc.? Now that your kiddo is 4 years old and is learning numbers at a faster pace, he is big enough to match numbers with it's symbols.
What you need to know:
At this point, your kiddo can identify what is the symbol of '1' and how '2' is different from '1' not in counting but also in symbols. Make it a daily practice, ask him to read numbers wherever he finds it around him to enhance his arithmetic skills further.