I have been running Ubuntu 16.04 on my old Dell XPS 9530 with 4K screen, with an external regular DPI screen, and it has been almost OK. I can set scale individually on each screen - and set what screen new apps should scale for.
Now I have a new Dell XPS 9570, also with 4K screen, and I wanted to go for Ubuntu 18.04 (for the best drivers). And I discover to my surprise that I can only scale either 100% or 200% (or 300 / 400%) and the setting is applied for ALL attached screens, not pr screen as for Ubuntu 16.04. Is there some kind of way to make this available on a pr screen basis? I just looked at xrender, and it does not look like anything I want to touch. (My time is unfortunately hugely limited)
Other solutions I've found are setting the 4K display to 1K resolution. (I'd like to avoid this solution)
Am I the only one with a 4K laptop screen running an external regular-DPI monitor? (Perhaps this is my queue for a monitor-upgrade?)
Don't mean to sound negative wrt Ubuntu 18.04 - it looks great and I love using it - as long as I don't attach an external regular-DPI screen to it...