Here's a photo of a Seiki 50" Ultra HD (4K) TV running in HiDPI (retina) mode off my rMBP:
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Only problem with using it as a monitor is the slow max refresh rate of 30Hz. The mouse jumps across the screen and theres a lot of tearing when moving menus around. This is a limitation of HDMI 1.4a that only supports a max of 30Hz at 4K resolutions.
And you couldn't effectively use it for anything more than playing games, surfing the web, or watching movies. Color accuracy on TV screens is iffy at best. So that definitely takes Lightroom, Photoshop, or any other design tool out of the mix.