The Raspberry Pi 4 is a nifty bit of kit but there have been a few issues with getting a signal out of the HDMI port. It may seem to boot but there will be no signal to the screen.
Firstly make sure there's an operating system installed on an intact MicroSD Card. If there's no SD Card inserted or the card is corrupt, the thing won't boot. If you're using Raspbian, the Pi4 needs the Raspbian Buster version or later so download it and perform a fresh install.
Before booting to the fresh install, edit the MicroSD Card and enable SSH login. This lets you configure the RasPi from another system and is always recommended.
Note the Pi4 has two Micro HDMI ports and for a single screen you must use the left one (hdmi0). The system is not designed to boot to a single monitor using the right hand HDMI port (hdmi1).
You will probably need a MicroHDMI-to-HDMI converter to connect to the screen. Choose a relatively skinny one as it sits close to the USB-C port and I had to cut mine down so they could sit together nicely. Brutal measures indeed..
Put the freshly installed MicroSD into the Pi and boot the system, making sure the card and all cables are inserted fully.
If nothing comes up on screen (and make sure the screen's powered up ) and everything's plugged in properly, you're gonna have to edit a file on the MicroSD Card. If you know the IP Address of the Pi4 you can SSH in and edit the file (assuming it has actually booted), otherwise power down and put the MicroSD Card into a card-reader on another system.
The file you need to edit is /boot/config.txt
Enter (or uncomment) the following line:
This will force display through HDMI, even if no HDMI screen is detected; it fixed things nicely for me. Save and exit from the text editor and reboot the system.
Other options include also adding (or uncommenting) the following line:
.. as using HDMI mode rather than DVI mode can solve some sound problems.
If it's still not working, recomment the above lines and try:
This forces the Pi to use maximum HDMI compatibility mode but may result in average display quality. Still better than nothing for now eh..
If it's still not working, and mate, I'd be severely crappin' my strides by now, make sure you're not using some shitty little power supply left over from your Nokia brick days. The Pi4 has higher power demands to feed its dreadful soul and needs something like 5.1V/3.0A DC so don't scrimp on the juice..
AndyM | September 2019