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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

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.