The 4K monitors have resolution of 3840×2160 and they are already on market. I believe the cheapest contender is 39" seiki[1] which goes for under $1k.
For more dots per inch there is bunch of 31.5" 4K screens coming to market at the $3-4k price range.
Except most 4K displays can't do 60Hz which for most is the bare minimum for computer usage.
I would avoid that Seiki monitor, it can only do 4K at 30Hz max and there are wide reports that it can't even do 1080p at 120 Hz properly (even though its in the spec sheet).
The first 4K 60p input displays are just coming to the market this year. You can see a list:
http://www.noteloop.com/kit/4k/
Right now it is just a trickle but expect a flood of both cheap and expensive 4K 60p displays next year.
For more dots per inch there is bunch of 31.5" 4K screens coming to market at the $3-4k price range.
http://4k-monitors.com/seiki-39-se39uy04/