The other day I was explaining to one that their designs fixed width looks silly once it got up towards 4k resolutions. But the designers main concern was if people actually used full screen browsers on 4k monitors and if there was any point in thinking about the design at that resolution.
There are plenty of times I enjoy have 2 browsers side by side of even 4 browsers in a square, and being able to do that is one of the benefits of having a 4k monitor. But without a doubt the majority of my time is spent with a full size browser window open, and I observe the same from all the other windows/linux users I manage that use a 4k monitor.
In service of keeping this post simple, I've ignore system display/ui scaling. But still... the question/assumption from the mac designer completely blew my mind.