If you’re using Google Photos or Flickr to host videos, your videos have already been re-encoded to an MP4 format for use across pretty much all devices. But if your video is on Google Drive, Dropbox, or OneDrive, you are in charge of making sure your video is compatible with your playback device.
MP4 files support a variety of resolutions, codecs, and encoding levels. Your device needs to support these in order to natively decode the video.
If you’re trying to play the video on a Fire TV, the list of supported formats by the decoder is at https://developer.amazon.com/docs/fire-tv/device-specifications-fire-tv-streaming-media-player.html
For example, the 2014 Fire TV does not support decoding h.265 encoded videos and only supports h.264 encoded videos at 1080p and 30 fps. If you try to decode an h.265 or 4k video, it will fail. The device doesn’t understand these encodings.
In general, you’re going to have experience problems decoding and rendering 4K videos on 1080p devices. These videos are best re-encoded to a resolution and bandwidth that aligns with the device’s native resolution.