Based on VisaJourney.com information provided on the website:
Briefly, the K3/K4 process is as follows:
A. The US Citizen files an I-130 petition for the foreign spouse.
B. USCIS (INS) issues a receipt (I-797) acknowledging filing of the I-130 petition.
C. The US Citizen files an I-129F petition, using the I-797 receipt as "proof of filing an immigrant petition". All children of the foreign spouse will be listed on this petition. The I-129F is sent to the Service Center where the underlying I-130 petition is currently pending. At this point, the "track" of the K3/K4 process becomes similar to the current processing for K1/K2 fiance(e) visa.
D. When the petition is approved, it is sent to the NVC for processing and then the appropriate Consulate is notified and processing for the K3/K4 non-immigrant visa is begun, including medical, police check, I-134 affidavit of support, and Consular interview. The fee will be the same as the current K1 fiance(e) visa.
E. The K3 visa is issued to the spouse, the K4 to the minor children, they go to the United States.
F. The K3 spouse and K4 children file the I-485 for Adjustment of Status, either immediately or when the I-130 petition is approved. The new K visa will be issued for an effective period of 2 years, with multiple entries permitted, and provision to extend the visa beyond 2 years. The State Department has indicated that in K3/K4 cases, that the USCIS (INS) will retain the approved I-130 petition. This means if he K3/K4 visa holder wishes to obtain an immigrant visa at a Consulate, they need to notify that Consulate to begin the process, after which the Consulate will request the approved I-130 petition from the USCIS (INS). The USCIS (INS) seems to expect that most K3/K4 folks will apply for AOS from inside the United States, but due to long processing times at some local offices, it is very possible some folks will wish to opt for Consular processing after they arrive in the United States.
Note about the I-130 approval - In some cases the I-130 may be approved prior to the K-3 being processed. There are certain advantages if this happens such as receiving an Immigrant Visa instead of a K3. Please read the IR1/CR1 Guide if your I-130 is approved during the K3 Process to find out what you can do.
http://www.visajourney.com/faq/k3k4visa-outline.html