Well, a 4k image is 4 x 1024, yet you trying to run a 1080p image full screen, its not going to be very sharp. It gets worse the smaller the source image is, since going to 4k its reduced the number of sizes I consider reasonable to watch as anything smaller than 720p is a blur fest at full screen.
Some tv's fight this by upscaling the image to match 4k
the 4k we get on monitors isn't true 4k, it only 3840 by 2160 whereas it should be over 4000 by 2160 but its close enough to make any smaller image look bad.