The Panasonic GH3 became incredibly popular because of it's best-in-class video capabilities, and the GH4 takes it up a notch with adding capability to shoot 4K video. Yep, a $1700 camera capable of shooting 4K video at 24p or 30p.
On the surface, it seems just like every other 16mp m4/3rds camera system, but beneath the magnesium body is a new sensor and fast processor which can not only shoot 4K video, but also shoot 8.8mp photos during video capture. In addition, it has an upgraded shutter system capable of lasting for over 200,000 shots, with a speed of up to 1/8000 of a second.
It has both an OLED viewfinder and rear display, 4:2:2 HDMI output in 8 or 10 bit, and microphone inputs with a headphone monitoring port as well. This is a true videographer's tool capable of keeping up cameras at least twice the price.