You need to set it to Best in order to get a 4K stream. It will still drop the stream quality as needed according to what your connection quality at the time dictates regardless. Setting it to a lower quality than balanced is simply a way of capping the bitrate in case someone has concerns about data usage.
Keep in mind that currently the only platform you can get a 4K stream on is via Chromecast, but support for this on other screens like the Chrome browser will be rolled out eventually.