I have an RX 460 in a 2010 5,1 Mac Pro it's driving my 4K monitor (TV) at 30Hz.https://www.amazon.co.uk/gp/product/B01J1J7WOM
As far as I can see there's no issues with the card whatsoever, the Metal API screensaver Rainingcubes works fine and for general desktop and programming use it is perfect, I don't use any audio/video editing software but this card is not powerful enough for any pro work anyway.
I was using an active adapter on an R9 280X to convert the DP output to HDMI 4k @60Hz, I was hoping the the HDMI 2.0 port would support 4K @60Hz but for whatever reason it does not, so I thought I'd try the DisplayPort but I do not get a signal from it at all.
Posted benchmarks for giggles...
GeekBench 4 Compute OpenCL
Unigine Valley - 4K Low Quality
Unigine Valley - FullHD 1920x1080 8xAA
Unigine Valley - FullHD 1920x1080 No AA (NB Locked to 30Hz/30FPS anyway..)