S4-SGSN is an evolution of the SGSN (Serving GPRS Support Node) used in 3G networks. This evolution allows the SGSN to integrate with 4G-LTE nodes (the SGW and the PGW).

The S4 interface is the interface between the S4-SGSN and the SGW. It allows the SGSN to interact with the S-GW for the establishment, modification, and termination of user sessions.

The S4-SGSN is responsible for managing the user sessions, including the establishment, modification, and termination of data sessions. It works in conjunction with other network elements, such as the Serving Gateway (S-GW) and the Packet Data Network Gateway (P-GW), to control and maintain the user sessions.