Unfortunately, it takes at 8.4 volts only 1A, which is too little for a (external) supply a GH4 - she moans .. and switches off after some flickering.
Unfair I find the 20,000mAh indication, because this is true (we learn only in Handbüchlein!) At 3.7 volts! At 8.4 volts, it will then be estimated 8.000mAh :-(
Presumably, the Power Bank will decline - a pity - it will try with an anchor 20,000mAh that supplies 9 volts 2 A, which was also a rich GH4 (hopefully ;-))