How to disable wifi in Raspberry Pi 4

Use rfkill block wifi and reboot to disable and rfkill unblock wifi and reboot to enable it again.

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.

Create /etc/modprobe.d/brcm-blacklist.conf with the following content:

blacklist brcmfmac
blacklist brcmutil

Reboot, and WiFi should be disabled. You may find errors in the system log depending on what method you are using to configure the network. You can choose to correct those, or not, of course.