Do they have enough oompf for anything other than being a door stop?
We have one at work we're using as a display station, and it is passable, but don't try remoting in to it. I know enough now to make the connection and go for a coffee while I wait for the first screen refresh.
Personally I'd buy a low end i3 and a mid line graphics card or look for a mobo that has an upgraded integrated graphics chipset. I think Zotak actually makes a couple of units like that.
Are the 4K codecs all defined? What sort of processing power is required for them?