as an overall expereince 4 is a lot better, but mechanics wise 6 improves on 4 in nearly every area. 6 isnt as well designed, but its still good. just dont expect RE4 level quality.
Mechanics were not improved, they were changed. Quick turn doesn't work properly and shouldn't even be in the game. The camera in RE6, although given control over it, is more of a hassle than a help. With the old camera I felt I had much more mobility and awareness of my surroundings with the properly functioning quick turn. RE6 quick turn is misnamed. It's a quick character turn, you still have to move the camera.
You can dodge, slide and flop on your back now. Pretty much pointless additions to the game that aren't even necessary to use except in a few scripted parts. Moving and shooting are implemented, but there's no need for it either since you're moving at Tyrant speed.