H-4 visa stamp

Anybody knows how to get a H-4 visa stamp (or any other visa stamp) on a passport in the US Embassy in Beijing? We need to go back to China for a family emergency immediately. We have changed our visa from J-2 to H-4 in the US, but without a stamp on our passports. I was told that we need to obtain H-4 visa stamps on our passports, in order to re-enter the US.
I deeply appreciate your information about how to get it and how long it will take. Many thanks! Email: [e-mail]