Packet 3 is sent by Delhi and Mumbai Consulates. People are required to send back a checklist and applications to show they are ready for the interview. Then the interview is scheduled.
Packet 4 is sent giving the interview date and instructions for the interview and medical, etc.
Chennai Consulate schedules the interview immediately upon getting the case from the NVC, they do NOT send out Packet 3. They only send Packet 4 giving the interview date. We went through Chennai Consulate so that's all I have experience of. The contents of Packet 4 were as follows:
1. Domicile for IV sponsors
2. DS 230
3. DS 156
4. DS 156K
5. Information for K1 Applicants
6. K1 Interview Appointment Letter
7. Medical Exam Cover Letter & Instructions
8. Photo Instructions & Poverty Guidelines
9. VFS Addresses
Most of this information is available on the Consulate websites.

(If anyone is interested in getting the above document attachments, just send me a PM with your email address.)