Question: What triggers the increment of the `SM.N4SessionEstabFail` counter, which measures the number of failed N4 session establishments at the UPF?
Option 1: Receipt of an N4 session establishment request message by the UPF from SMF.
Option 2: Successful transmission of user data over a established N4 session.
Option 3: Periodic timer expiration for N4 session maintenance.
Option 4: Transmission of an N4 session establishment response message with a rejection cause by the UPF to SMF.
Explanation: The `SM.N4SessionEstabFail` counter increments upon transmission of an N4 session establishment response message containing a cause indicating rejection of the N4 session establishment request, signaling a failed establishment due to a specific cause.