Just discovered that some GUI interfaces seem also slightly blurry, like Toontrack EZkeys, while others are OK. Reading on forums it seems a lot has to do with graphic cards drivers not coping with 4k HDMI that well ....

At least i was advised replace the AMD Radeon r7 360 (with HDMI port) is not too compatible, and get a Nvdia 1050 Ti as minimal possible solution instead.