There are some ACSY and XVTR ports on the back, but I couldn’t reliably determine the audio lines if they are present. A less attractive, but known to be working approach is interfacing through the mic and headphones jack:
GX4 pin 1 → 3.5mm TRRS ring 1 (RIG_AFIN)
GX4 pin 2 → 3.5mm TRRS ring 2 (RIG_PTT)
GX4 pin 3 → 3.5mm TRRS sleeve (GND)
GX4 pin 4 → 3.5mm TRRS sleeve (GND)
1/4" TRS tip → 3.5mm TRRS tip (RIG_AFOUT)