Raspbian is managing hardware with overlays. In `/boot/overlays/README` you will find:

    Name:   pi3-disable-wifi
    Info:   Disable Pi3 onboard WiFi
    Load:   dtoverlay=pi3-disable-wifi
    Params: <None>

I suggest you just add

    dtoverlay=pi3-disable-wifi

to `/boot/config.txt` to disable wifi. Because I have taken this from Rasbian Buster I assume it is also valid for Pi4.