Possibility for "Custom" setting of resolution and bitrate
I may have missed it, but I cannot find any possibility of publishing with a custom bitrate in the Video settings for .mp4
Coupled with that would be to have 4K resolution available.
Photopia does not currently have an option to create video output with fully customized parameters, although it's something we hope to see very soon. In the meantime, there are actually a few places within the Publish window where you can find a 4K profile. Look under "Video File" > "AVCHD", for instance, and you should see a "4K UHD" option located therein.