The newer OSes out there now support 4K or "retina" resolutions. Unfortunately they've chosen to do it in a slightly hacky way in order to hide the details from applications to improve compatibility, and of course, it has had exactly the opposite effect!
If you're finding that you can only see a quarter of the game screen, you will need to tweak the application compatibility properties.
Under Windows, it's "Disable display scaling on high DPI settings" (set it on the .exe in Explorer)
On Mac OS X, it's "Open in Low Resolution Mode" (set it from the app properties in the Finder)
Also of use is using Alt-Enter to toggle fullscreen/windowed mode.