N4 interface in the 5G network is responsible for the interaction between the Session Management Function (SMF), and the User Plane Function (UPF). And it uses the PFCP protocol.

The SMF uses the N4 interface to select the appropriate UPF for a specific user session. And to establish, modify, and release user plane sessions between the SMF and the UPF. And to configure the selected UPF with the necessary rules and policies for handling the user's data traffic.

On the other hand, the UPF can report session-related information and measurement data to the SMF over the N4 interface. This data can be used for charging and monitoring purposes.