I have a 4 month old when is it safe to have another baby?

There is a difference between safe and smart. It is probably safe for you to have another chlid now. One of the problems with having another child so quickly is that being pregnant will mess with your milk supply for your current child. Granted you'll be starting with solids soon so it might not be that big of an issue.

It is smart, if you want to have them quickly, to wait until you've weaned the first child. Though it could take up another year to become pregnant again.