If your Firestick 4K's power button is not turning off your TV, here are some things you can try to fix the issue:

Check if your TV supports CEC: Check if your TV supports Consumer Electronics Control (CEC), which is a feature that allows you to control multiple HDMI-connected devices with one remote control. If your TV supports CEC, make sure that the feature is turned on in both the Firestick and your TV's settings.

Restart the Firestick: Sometimes a simple restart can fix the issue. Go to Settings - My Fire TV - Restart to restart the Firestick.

Check HDMI-CEC settings: Go to Settings - Display & Sound - HDMI-CEC and ensure that the "Device Control" and "Power On" options are turned on.

Change HDMI port: Try plugging the Firestick into a different HDMI port on your TV and see if that resolves the issue.

Check power supply: Ensure that your Firestick 4K is powered using the supplied power adapter and not just the USB port on the TV. If the power supply is insufficient, the Firestick may not be able to properly control the TV's power.

Try a different remote: If you have another remote that can control the TV, try using it to see if the issue is with the Firestick remote.

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