What do I have to do to change my 4year old son's last name?

If you want to do a name change the birth father has to consent to that.  If the birth father is not named on the birth certificate, you might be able to legally do the name change without him.  If you are not getting child support from the father and don't know where he lives or how to get in touch with him, you may be able to file the paperwork without his consent.  If you do know where he is, it would be best to get his consent.  The paperwork you need to file can be found at the New Jersey Judiciary website.