Here is a copy of the K3/K4 Interview Preparation Instructions. This is sent to a K3/K4 applicant once the I-129F visa petition approved by the USCIS and sent to the US Embassy.
Dear K3/K4 Visa Applicant:
This is to inform you that an approved I-129F visa petition has been received by the Embassy from the U.S. Citizenship and Immigration Services (USCIS) according you nonimmigrant status as the spouse of a U.S. citizen (K3) under Section 101(a)(15)(K)(ii) of the Immigration and Nationality Act (INA), as amended. This allows you and your unmarried minor child(ren) (K4) to enter the United States as nonimmigrants and later apply for adjustment of status as permanent residents. You are now eligible to be scheduled for a visa interview. You must pay the K visa application fee before you can schedule an interview appointment.