He's not going to be ready for cow's milk until he is at least a year old. Cow's milk is one of the biggest allergens out there (along with soy). This is why the American Academy of Pediatrics recommends breastfeeding for a minimum of one year (their policy can be seen
here).
I don't mean to seem like I'm coming down on you.

I just want you to realize that introducing milk to early can lead to troubles later on (babies' tummies especially aren't made to handle milk).
Four months is a great start, though! Welcome. We have the same overall goal weight.