Yep, that's the problem. For whatever reason, you are forced to use 6-digit user codes if you enable the C4 interface. Switching back to 4-digit codes turns off the C4 interface.
On the older IQ2+ panels, you used to be able to switch back to 4-digit codes and the C4 interface would remain enabled, but that was changed in a subsequent firmware update. That was never possible with the IQ4 though.
