UI3's streaming profiles with "VBR" in the name are configured to use relatively low quality and low bit rate limit in order to work more efficiently over slow or data-capped networks.
If you have the bandwidth to spare, the non-VBR profiles such as "2160p" are much higher quality.
Specifically the 2160p profile defines that the stream should use:
- Quality: 50 (you will be tempted to set quality to 100 but doing so will not help meaningfully, and may cause stability problems)
- Max Width: 3840
- Max Height: 2160
- Max Bit Rate (Kbps): 8192
You can edit this and other profiles via UI3's main menu > Streaming Profiles: