My fs100 batteries would last me 2-3 days of shooting, like at WCS I'd charge a battery every two days, solid 4-5 hours of shooting per day, and I'd have like 20-30% battery left but I just didnt wanna take my camera up the next day with that low of batts.
My gh3 batteries are pretty solid too, but id say a batt would last one day of shooting, itd probably be 2/3 done and could bleed into the next day, but not worth risking/if you can charge your battery why wouldnt you?
As for fs100 vs gh4 thats an tough one. Honestly, if you don't need 4k I'd say fs100. Im not sure who was saying it, but the fs100 is NOT dated. It shoots 1080p60 and other than not having s-log, it has everything you need. I loved my fs100, shit I almost bought one a month ago but realized I didn't need it haha. The gh4 does have some mego mo options, if thats something that tickles your taint, but in reality you could find an fs700 for like $3k and that would be the BEST option IMO.
Also consider an a7s, with the mark 2 coming out they will be easy to find for cheap, no 4k but a comparable camera to a gh4. If you really want a pani/lumix camera wait for the gh5.