Apple TV has a really low datarate. Even compared to other streaming services, who regularly chimp out on bandwidth as well (e.g. youtube, netflix).
There is no benefit in selecting "4k" as it is optically less than 2k, like you said.
Netflix seemed to have cranked up the bandwidth lately. Some newer shows really start to make use of the Full HD resolution. Maybe 4k will make sense soon for commercial video (except adult stuff - they have always been high end).