The U-series of Haswell CPU-s only support 30hz for 4K resolution. Which is borderline unusable. However, using reduced blanking timings, it should be possible to get 50Hz. I have a ubuntu laptop with i5-4200U and after a couple of hours of googling found the link above. Looks more complicated in linux than on windows, where Intel provides a tool with its drivers. Disclaimer: have not tried yet.