4K videos are usually encoded in HEVC/h265 which is more difficult to transcode on the CPU. That’s why you will need a modern Mac and a good network to stream 4K movies.
4K content usually has a higher bitrate than the other content, so make sure that you have enough bandwidth to stream 4K files in their native resolution. If you need to transcode the video to 1080p there is no use in 4K.
Make sure that your receiving device can natively play 4K format without transcoding it. If you need to transcode the video, any quality you had will be instantly lost. Native playback of 1080p/720p will be a better solution than transcoding 4K.