Note than when comparing TV/monitors, you MUST compare the pixel pitches of each, which are usually listed in the device specifications.
With OSes, like Win/OSX, that specify font and GUI elements in pixels, the size of dialogs, text and other display elements is directly proportional to the pixel pitch for that display device.
If the displays are scaled at 100% in the OS, then text/control/dialogs displayed on a Seiki 50" 4K TV (pixel pitch = 0.265mm) will be 15% larger than on a 27" 2560x1440 monitor (pixel pitch = 0.230mm). Conversely, a 4K TV/monitor would have to be 40.5" to have the same size pixels as that 27".
Now please stop the FUD about how 4K will make everything SO SMALL, and get back to reality!