Just got a new Mac delivered (primarily work machine) but was really interested in trying Zwift on it just for the sake of it:
Mac Pro (2019)
CPU: 3.2 GHz 16-Core Intel Xeon W3245
RAM 48 GB 2933 MHz DDR4
Graphics: AMD Radeon PRO W6800X 32GB
Surprised to see Zwift showing only 1080 from the options. Please no mention of $30 Intel i5 or i3 processors, that’s not what I’m flagging here. Appears to be that this card might have been missed -it is way more powerful than my old RX580 which does get 4K Ultra option.
It’s a manual lookup. Tons of workstation GPUs aren’t accounted for, on the basis that hardly anyone uses them for gaming.
I logged this with support to see if they can get this passed over to developers to have the profile added for Windows and macOS versions of Zwift.
Hopefully it’s a quick fix. This card and computer should run 4K ultra very easily. Even tweaking the config settings does nothing.
I should have suggested including W6900X 32GB and W6800X Duo 64GB as well.
I ran Microsoft FlightSim 2020 on same machine and was amazed, max settings (looking like a photograph) and 45fps at 4K. 
I know Zwift isn’t so well optimised however.