A little mysteriously, Panasonic's description of the update (download it here) is limited to: "Improved [sic] the reliability of video recording performance."
Perhaps this is to reduce the incidence of crashes? My GH3 would crash sometimes (not often), but it was a PITA. I haven't shot with the GH4 enough to crash it, but, in his review, Dave Dugdale does mention a few crashes he had with his test camera. Hopefully, upgrading to firmware v1.1 means that my new GH4 will never crash :)