I can only speak to the audio side of your project. I am a sound mixer in film/tv and recently did my first Facebook Live video - if you go to the Dragons Den FB page and look for the Manjit Minhaus clip you'll see how it turned out. edit:
https://www.facebook.com/dragonsden/...5770519484988/
For this I used all my pro gear, but then interfaced with an iPhone using a iRig Pro. The iRig is mono, and because I only had two people on camera and it was unscripted I mostly just left both their mics live - potting either down whenever it felt like one would be talking for a while. The iRig also comes with a USB output cable allowing you to interface with a computer, so I think it could be good for what you're looking to do.
I'd definitely recommend trying to get a mic on each of your talent. Whether that's mics on stands in front of them, or a lav mic on each, it will make a huge difference to the quality of your production. If you are running a how-to style video where the visuals are most important, than people are more forgiving with bad audio. Interviews and the such need good sound or people tune out (unless it's the most riveting content ever).
Do you have someone operating camera/audio for you? Or is it a set it and forget it kind of thing? Ideally I would set up a signal chain in one of the following ways:
Talent Mics > Mixer > iRig> iPhone or Computer
Talent Mics > Mixer > GH4
In either scenario it would be great to have someone capable mixing on the fly. All it takes is one person to start coughing/scratching/fiddling and the noise they're making will be over everyone's audio if the mic is left open.
There are a few ways to interface with the GH4. You can get a cable that takes two XLR outputs from your mixer and brings them to a mini connector; or you can get a DSLR audio mixer similar to a Tascam DR60.
I could literally talk sound all day long, so feel free to reply here with any questions or take it to PM if you prefer.