ADS Simulation: Schematic to find input impedace
There is a wide tolerance on Vbr and Infineon's is Vbr= 4V min @ 100uA yet the typical log curve indicates 5uA at 4V with a slight shift in the log curve at 2.5V then again at 3.5V

Your signal shows a slight saturation above 3V and almost 10% compression at 4.5V


For Infineon the Absolute maximum Voltage rating is Vr=4V