We use cable to connect our device to controller. Using it, we can turn off WiFi on our device, so we can have less interference.
Connection between controller and spark, is something else.
That's the reason we try to fly in FCC mode, because this connection is stronger in FCC.
If everything is ok using WiFi instead of cable, I think you should try another go4 version, or another otg cable.
Don't forget to set your device in airplane mode, when you fly using cable.