Hey, I usually put movies on my 2TB hard drive and then insert it via the USB port on my Hitachi android smart tv. If a movie on my hard drive is in 4K or in BluRay 1080p, and then I play it on a media player like VLC. Does the media player actually play it fully in 4K or the BluRay 1080p that it is supposed to be playing, or is the quality reduced?
Since you are asking about 4K video, I'm going to assume that your TV is a 4K TV set.
You would need to check the specs for your TV's media player to see what it's able to do/support.
I did some looking around and for example a Hitachi 55HAK6150 Android TV, can display 1080p video with any of the video format that it supports, however it appears to only be able to display 4K video with only certain video formats (H.246, H.265/HVEC, and VP9), what is not clear is what happens if you try to play a 4K video using one of the unsupported video formats? Does it simply not play or does it downscale the video before playing? If I had to make a guess it would be the former and the video won't play.
For most folks, this limitation probably doesn't really matter, because most if not all 4K video is usually encoded using one of the supported video formats, because the other video formats would likely end up having a file size and bitrate that would be ridiculously large/high, but even so, the other video formats might be supported on a different or newer Android TV.
So, again, you would have to check the specs of your TV to see what it's able to do, because your Android TV's video support might be different.
There are other limitations when it comes to video and audio, but again, you would have to check the specs for your Android TV, to see what they are.
To recap, for 1080p video files, the TV should be able to play them and the TV would upscale the video to display it on a 4K display panel.
The bottom line it that the TV will either play a video or it won't play the video, and if it doesn't play, then you might need to reencode the video file into a format that the TV can play.
I hope this made sense and was helpful.
Best wishes and good luck!