Hi Paul,
Did you try to produce a video with a preset 3840 x 2160, High quality, 60p?
This file should be supported by your 4K TV.
Choosing High quality option or "Quality" mode as 100% (in the Custom preset) should give better visual results than setting a fixed bitrate mode. With "Quality" mode the encoder better and smarter uses video bitrate for different kinds of scenes. Also in Quality mode, the encoder will not exceed 60 mbit bitrate for better compatibility with a wide range of 4K TVs.