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.
- 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. There's a great official article on this here which explains the supported resolutions and how to tweak settings: https://www.raspberrypi.org/documentation/configuration/hdmi-config.md
Config.txt settings: With some models of TV you may have to undertake a bit of 'trial and error' with the config.txt video settings. There's a great Raspberry Pi article here detailing these settings: https://www.raspberrypi.org/documentation/configuration/config-txt/video.md.
Changing hdmi_safe to 1 is a good first step, then go from there.
For more info on config.txt, see this additional post: https://www.raspberrypi.org/documentation/configuration/config-txt/
- 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.