I have used this lens on my GH4, but decided to go with the 12-35 and 35-100 2.8 lenses instead, as I do a lot of low light jobs. It's a great all around lens for normal shooting, especially outdoors. It's for auction on ebay:
http://www.ebay.com/itm/401263835337?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1586.l2649
http://www.ebay.com/itm/401263835337?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1586.l2649