My setup is a mid 2010 27" iMac (11,3) with a Philips 288p 28" 4K external monitor, running OS X 10.10.2 at the moment of this typing. Managed to get 1920x1080 working in HiDPI on the Philips (based on 3840x1440@30Hz) which looks awesome. The old iMac can run its own screen at 2560x1440@60Hz at the same time. I'm looking for a way to match screen scaling however, which would be easier to work with in extended desktop mode. The options are: - to just run the iMac's monitor in 1920x1080 but that looks too blurry. - to run the iMac in 1920x1080 in HiDPI based on its native resolution - to scale the Philips to 2560x1440 based on 3840x2160@30Hz. Up until OS X 10.9 it was possible to create a scaled resolution in SwitchResX higher than the monitor's capabilities, which would result in seeing a HiDPI mode at 1/2 of that resolution. Running 10.10 the OS rejects any scaled resolutions exceeding the monitor's capabilities (at least for my graphics card, the AMD HD5750), which rules out the latter 2 options. Has anyone so far found a new way to create custom HiDPI resolutions in Yosemite?