Don't get an 4k monitor for altium! especially if you're using win10 (latest creators edition). There is no way to change the font size within Altium so you have to enable scaling (within windows) so you can read anything.. however this scales everything including the CAD area.. effectively robbing you of the extra resolution
I'm using a 4k monitor under AD18, with Windows scaling turned on. I still like having the extra resolution: it doesn't put more information on my screen, but it does give me a sharper image.
Also... scaling wreaks absolute havoc within the GUI.. menus don't show up correctly, some menus simply just can't be seen as they must appear outside the viewing area. At present I cannot access the Grid Manager as it doesn't display on the screen.
I have a 4k monitor and a QXGA (2560x1600) monitor side by side, with the 4k monitor as the main one. The 4k is running 150% under windows, with the QXGA running 100%. Running on either monitor alone, everything works just fine. (e.g. I can see the Grid manager which you mentioned.)
I do get odd effects when running Altium across both monitors in certain configurations. Using your Grid Manager example, I can see the grid manager on either monitor when the main Altium window is on the QXGA monitor. However, if the main Altium window is on the 4k monitor, the grid manager will work on the 4k monitor and not the QXGA monitor.
I hope Altium fixes this soon, but overall it's not a huge problem and not worth giving up the benefits of 4k.