If you're not seeing any HDMI output on your monitor or TV, here's a couple of suggestions which usually fix the problem:
- Wrong HDMI port: The Raspberry Pi 4 has two HDMI ports. The connector on the left marked 'HDMI0' is the primary/default display. If you're using just 1 screen you'll need to use this one.
- Re-seat your SD card: If your SD card isn't quite pushed in properly, the Raspberry Pi may show some lights but it won't be working properly. Remove power, take the SD card out, push it back in and try again.
- Re-write your SD card: Sometimes the operating system can fail on an SD card (even new ones) and needs to be re-written. If you have a card reader, you can follow this very simple process via the Raspberry Pi support page here: https://www.raspberrypi.org/downloads/
- Check Monitor/TV compatibility and settings - (also see the next item) Some TVs and monitors won't display a picture from your Raspberry Pi out of the box - you may need to check your TV resolution and/or change settings to make it work. We find this happens more with TVs/Smart TVs than monitors.
- If you can see the Raspberry Pi OS desktop but the resolution is wrong, you can change settings via the configuration tool. Select the Raspberry icon then head to Preferences > Screen Configuration to adjust settings.
- The official Raspberry Pi documentation details the different modes you can set in the Raspberry Pi config.txt file to help the Pi work with various displays/resolutions: https://www.raspberrypi.com/documentation/computers/config_txt.html#hdmi-mode
Config.txt settings: With some uncommon models of TV you may have to undertake a bit of 'trial and error' with the various config.txt video settings. The official Raspberry Pi documentation details these settings: https://www.raspberrypi.com/documentation/computers/config_txt.html#hdmi-mode
Changing hdmi_safe to 1 is a good first step, and then go from there.
- hdmi-mode can force the resolution your display needs too. For example, using the table here, we would try hdmi_mode=16 for a 1080p 60Hz 16:9 display.
- Check your monitor's input - sometimes we discover that customers had additional devices connected to other HDMI/VGA ports on their monitor, with the monitor defaulting to these devices rather than the Raspberry Pi. When troubleshooting, it's best to connect just the Raspberry Pi and remove all other connections to your monitor.