To me it looks like the old issue of a low bitrate at 4k and NR being applied. Whats profiles are you using? Personally I suggest +1,0,0. With sharpness at 0 or below it will add some noise reduction which will add muddy areas where there are blocks of the same colour such as in trees or clouds where the detail colours are similar.

thanks very much for the quick reply i'll try your suggestion . i use cine as standard nothing altered.