Also called: Ultra HD, 2160p, 4K IPTV.
To watch in 4K you need three things: content that is broadcast or encoded in 4K, a 4K-capable screen and streaming device, and a connection fast and stable enough for the higher bitrate.
On IPTV, 4K is usually offered on selected channels and titles rather than the whole lineup, so player apps show them in their own category or with a 4K label. Other channels play in HD or Full HD.
How does 4K UHD apply to SynexaTV?
SynexaTV streams in 4K on channels and titles available in 4K, on devices such as Fire TV Stick 4K, Apple TV 4K and 4K Smart TVs.
For background and history, see 4K UHD on Wikipedia.