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. I have tested it with a Raspberry Pi 4B. It is also valid for Pi4. When disabled you will not get a WiFi interface **wlan0** as you can check with `ip -br addr`.

**UPDATE:**  
*@JeffAtwood* [noted in his comment](https://raspberrypi.stackexchange.com/questions/100543/how-to-disable-wifi-in-raspberry-pi-4/100549#comment166502_100549)
>They renamed it to `disable-wifi` so you can use that, `pi3-disable-wifi` was kept as a synonym for compatibility. Same thing with disabling bluetooth.

2019-10-25: seems everything is documented now in `/boot/overlays/README`.