I have one, but I have it set to 1440p instead. The performance loss versus visual gains on a 28" thats only about ~1m from me just isn't worth it. There's also refresh rate to consider; most for under £500 have 60Hz or less. If you want high refresh rate AND 4K, you'll be looking toward the £1000 mark.
Also consider screen size and how far you'll be sitting from it. 4K on a smaller display will leave you with small UI elements in various games which might be hard to see.
I'd recommend 1440p if you're considering around the 27-28" mark. G-SYNC/Freesync are something to look out for, too. Higher the refresh rate, the better.