If you're using a 4K display with your MacBook Pro or Mac Pro and don't have excellent vision, you may want to hold off upgrading to Yosemite for now. 4K display support has taken a step back with Yosemite. A month ago I purchased a Dell UP3214Q 4K monitor for my late 2013 retina MacBook Pro. I have it running at 60Hz using the mini DisplayPort. The most recent releases of Mavericks (10.9.3 and later) improved support for 4K monitors. In the OS X Display preferences, you could choose among several scaled resolutions to increase text size and still have plenty of desktop space. 3008 x 1692 worked best for me. I upgraded to Yosemite last night. Sadly, all the scaled resolutions are now gone when I go to Display preferences. The only two choices available are native resolution 3840x2180 and 1920x1080. 1920x1080 look horrible and defeats the purpose of having a 4K monitor, so I have no choice but to use native resolution. Text and screen elements are very small at native resolution. If I go to General preferences, there's no option to increase text size. Fortunately I wear reading glasses, so I am able to read the very small text, but it's far from ideal. I really hope Apple will fix this in the next update to Yosemite and bring back the scaled resolutions!