I have a very simple sketch that runs on the M7, it simply boots the M4 core and then in the loop blinks the Red LED.
I have a sketch loaded to the M4 core to create an Access Point and perform some other work. When the M4 boots up I see a messages on Serial1 indicating it is attempting to create an access point using WiFi.beginAP. That call never returns so it is blocking on something.
The same Wi-Fi sketch works as expected on the M7 core.