I have a Surface 4 with Iris Graphics 540. I have been sufering for months with this annoying behaviour of 2nd 4k monitor going black randomly. I have tried everything (including Microsoft support) with no lucky.
Now.... finaly today it seems I have a solution:
I've set the monitor to DisplayPort 1.1 (it was set to 1.2 before).
I've been working for about 3h using video and pushing the cpu without having the black screen!
btw...driver version is: 21.20.16.4627
I have not yet tried with the latest driver (24....)
I'd recomment to give it a try an post your experience... hopefuly it will work for you too!