Visual Studio on 4k monitor
As Eric mentioned, the best solution is to set the monitor that you're going to have VS on as primary, and then log out and back in. It will then look "right" on that monitor and fuzzy on the others. I agree that it's ridiculous that they haven't fixed this yet.
Update: right after I typed that I updated VS 2017 to 15.3.3, and it appears to be fixed! Looks fine on all monitors now.
Update2: Never mind, it still looks bad if I set a different scaling amount as my primary.
Unfortunately Visual Studio is still not per-monitor DPI aware. If you disconnect second monitor everything should work fine. The only way is to find some DPI that will fit for both monitors, but as you have 4K and 1080p it can be very difficult.
I have similar problem (not 4K, but still running 200% DPI) and at the end I'm using VS only on one monitor.
It is pity that Microsoft didn't solve this problem even in VS2017.
Another option: You can add a registry entry to make Visual Studio DPI unaware.
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\devenv.exe] "dpiAwareness"=dword:00000000
The above way is kind of “undocumented”. I did some research but couldn’t find any proper documentation about the dpiAwareness REG_DWORD in the Image File Execution Options key. Setting the REG_DWORD to 1 will make Visual Studio DPI aware again (requires a restart of Visual Studio).
Note: when Visual Studio is DPI “unaware” and display scaling is in effect, fonts and icons may look blurry but the WinForms designer will have no unwanted side-effects.