Non-recommendation: I just got a Dell S2817Q 28" 4k display to try with my Hackintosh box in the hope that it would also work with my 17" early 2011 MBP at 2560x1440 (2 DisplayPort inputs was a selling point). Long-term plan is to switch to a Fabled New iMac + MacBook combo and use the cheap Dell as a second display for both.
Sadly, while it works fine with the Hackintosh (in glorious 4k), so far I can't get my MBP to display anything (an older 2010 MBP will do 1024x768 but nothing more). EDIT: Workaround found - see later post.
Both the MBPs are on Mavericks, so I don't know if a newer OS would do better (but they're still on Mavericks for a reason).
Also, even if it works, the display quality on an old Mac driving a 4k screen is going to be poor c.f. a native 2560x1440 screen. I've tried forcing the Hackintosh to 2560x1440 "low-resolution" mode* - which is what a non-4k Mac would do if it worked - and the result is rather soft and fuzzy compared with a true 1440p display. Perfectly usable, but not optimal - and that's at 28" - it's going to be even more noticeable on a 30-43" screen.
My take-home lesson is to be patient and wait until you have a 4k-capable Mac, or your display choice is going to be hobbled by the need to support the old MBP and you'll be living with sub-optimal displat quality until then. (If you're desperate, 2560x1440 displays are cheaper and very not rubbish).
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Apart from that, on the Hacknitosh, the S2817Q display is great for a "budget" 4k (slightly muted colour, but I quite like that - I'm not a fan of Kodachrome Gold) and 28" is about the sweet spot where 4k native res starts to become just about usable, and the scaled modes very usable. Downsides are the lack of a swivel/height adjustable stand which also contributes to the cables being very hard to plug in and a genius bit of bad design whereby the latch on a latched displayport cable faces the casing and is the devil of a job to undo. The menu buttons are really fiddly, too.
*On a 4k-capable machine, you have to option-click on "scaled" then check "show low resolution modes" and select "2560x1440 (low resolution)" to simulate what you'd get on a 2010 MBP - everything rendered at 1440p and then scaled up by the display. Otherwise, in the high-res, scaled, "looks like 2560x1440" mode, the screen is still being driven at 4k, most of what you see is being rendered at high resolution and the result is better than a native 1440p screen. The difference between that and "2560x1440 (low resolution)" and is night and day.