4.0 improved AF for my X-T1 dramatically. Low light misses are just not there anymore. This is with the 60mm, 14mm, 18-55mm and the 55-200. The 55-200 was always a problematic lens before this firmware. One hit, solid, and shoot.
I have to say, thanks Fuji, well done.
Still would love to see iso 100 in raw, but I guess for some reason that just can't happen.
My only issue with the camera, the fact that early versions have the flush buttons on the back around the menu button, which don't give a very tactical feel. Something Fuji did fix on later versions of the X-T1.
Paul