First get a small Cage from Wooden Camera. It is the most adaptable and won't add much bulk to your system, though allows you to add a few things when needed, especially a top handle for dutch angles.
Second buy good nd faders to use as your f stop. Tiffen test out well.
If your using the 2.8 zooms remember in reality at wide open your at 5.6, so for subject isolation try to step back when you can and add a few more feet in focal length (but that's an artistic decision).
For sound the GH4 has lousy shielding and pre amps, so you can live with it, or buy a tascam that you run from mic to tascam to camera. That will help dramatically and the tascams are cheap.
For mics I've got em all and the radio Senihauser Lavs I find are the best, also their microphones. They make a small mic that is fairly fragile but mounts on camera and is good for foley.
For one person interviews use a radio lav and get some pads that go between the body and the mic. This cuts down on rubbing and clothes crunching noises (you can find them on google).
For multi person interviews either add another lav, or if the subject(s) are stationary and you don't have a boom operator mount an adjustable directional mic (with windscreen) on a stand and point it out of camera frame to the subjects. It works very well.
I have Rode, don't like most of their mics and most people go with Seinhauser. That's your call.
I would get battery powered rather than phantom powered mics, if you can.
Remember you can fix an image (sometimes) but rarely ever fix bad sound.
For WB never set it on auto when filming as it will change as conditions change. You will need to do some testing for coloration and tone out of camera as the gh4 does not have an S-log or a technicolor log.
Try to get somewhat flat and hold the highlights as much as possible, manually adjust the color temp. Keep reference files so you footage will match as close as possible.
Adjust your screen (at least the brightness to more match your computer so your not surprised when you shoot).
You can move the color settings around in the menu and you'll probably won't to remove as much yellow and red as possible as panasonic sets their file to that bias.
Unless your working to a separate hdmi recorder, the file is 8 bit so it's somewhat fragile (though the best of the h264 cameras I've used). Do some tests where you set your file to the camera's lcd or viewfinder then put it in the computer and see how it holds the highlights. Usually whatever you set in camera, you then stop down 1/3, but that's very user dependent. If you stop down too far your going to get a lot of noise.
You'll probably work 4k because most people believe bigger is better, (sometimes it is), but the bps on 2k is heafty and I'd shoot 2k, only go to 4k if their is the possibllity of alaising or moire.
But that's a judgement call.
I just got a Panasonic GH4, for very short interviews and documentaries, and am starting to find my feet - expect it will take 6 months at least.
First glance at my footage, white balance is the only real issue. Any advice?
I'm thinking of adopting Resolve Lite in due course, because it's free.
Paradoxically, after my first tests, sound worries me more; I cannot figure out how to do clean sound on camera, and think I will go with Sennheiser HF and lav.