With the Apple TV 4K (3rd generation) you can play videos from an external hard drive, but there are some limitations and requirements:

## Video formats
The Apple TV 4K supports various video formats, but not directly via an external hard drive. It supports formats such as H.264, HEVC, MPEG-4, but not MKV by default.

## Apps for video playback
You can download apps like Infuse that support MKV and other video formats. Infuse is a popular app for the Apple TV that can play a wide range of video formats, including MKV.

## External hard drive
The Apple TV 4K does not have USB ports, which means that you cannot connect an external hard drive directly to the device. You will therefore need to use other methods to transfer your videos to the Apple TV, such as over the network or by uploading to iCloud or other compatible services.

To summarise: You can use apps such as Infuse to play MKV files and other video formats on the Apple TV, but you will need to transfer these files to the device via the network or other compatible methods, as a direct connection to an external hard drive is not possible.