According to the Panasonic USAs rep talking to the guys at Zacuto the improvement is sensitivity is only about 1/3 stop in the real world (marketing material makes claims almost a full stop improvement). However the venus engine is also improved so video at higher ISOs should become much cleaner looking nevertheless thanks to better noise reduction on the sensor read out data. But for a massive light sensitivety improvement you will need to wait for future cameras with the pending organic sensor technology, Gh5 or GH6 perhaps. ;-)
Also from the Zacuto interview; on the GH4 you could get real "video" settings like "dB gain" and "shutter angle" in the menu system instead of the DSLR orientated "ISO" and "shutter speed" controls. This makes the gH4 an even more video orientated camera than ever before.