Exactly.
What is it that you're actually looking for in a display?
Most of the 4K displays available right now are the wrong size for how OS X handles scaling - only rendering at 1x or 2x, when Windows lets you choose between 100%, 125%, 150%, and 200%. (125/150% being the best options for your Sharp display)
Ideally a 4K monitor would be either 22″ (1080p HiDPI mode) or 44″ (3840x2160) for OS X.
If you're wanting a lot of workspace, something like
Panasonic's TC-L65WT600 might be more suited.
It's a bit larger than ideal, and at ~70 PPI items will be roughly the same physical size as HiDPI mode on your Sharp display. You would have to sit further from it, so that would probably be a good thing.
The problem is that 4K is mostly limited to either 30Hz, or 60Hz with reduced color resolution (4:2:0 color on Sony's latest displays) on the larger displays right now, and this is one of the few options which will do full resolution 4K at 60Hz for a reasonable price.
Here's one being used as a monitor:
[url=http://i.imgur.com/ia0Yg7E.jpg]Image[/url]