Does DS920+, DS720+, DS420+ and DS220+ support streaming 4K videos?
It depends mostly on whether your device supports 4K streaming or not. Most modern multimedia devices can play 4K videos smoothly. Devices such as smart TVs and multimedia players can access videos stored on Synology NAS using My Network Places (SMB) or Media Server (DLNA). You can also use Video Station and its corresponding mobile application, DS video, to play videos on your browser or mobile device.
Synology NAS will provide a highly compatible video format. With this they mean that Synology will transcode (downgrade) your 4K video file into a compatible format for your TV if video is not supported.
You can also use offline transcoding option to so so.