Question on quality:
GH4 internal audio does a pretty decent job. How much better would the quality of routing through an external recorder like Zoom or other be in comparison to simply recording direct into camera via an impedance transforming XLR to 3/8 Mic? Is the Zoom H5 (or other) pre-amp/quality benefits worth the hassle?
I recently invested in DPA lavs and Zaxcom Maxx recorder for my audio kit and I just have to say its WAAAAAAAAAAAAAAYYYYYYY better. But its something you'll not notice or worry about until you try it. My colleague that I helped on a shoot, I used my new kit and he calls me up saying wow man, I didn't have to touch the audio, it sounded amazing. The noise floor is so low its almost like iZotope RX has been applied (without any artifacts). The vocals kind of take on a 3 dimensional quality. But its a lot of money and I had just prioritized it since cameras and such are always coming and going, there really isn't much that'll get dramatically better in the audio kit when you get to that level, so I saw it as a much better investment to increase my quality. Next will be a new camera or two...
In regards to your specific question the zoom, yes it will sound better since zoom does have better preamps and a/d conversion. But can you use the other, yes, will you notice its worse...probably not, unless you recorded the same thing at the same time with a higher quality mic and recorder rig. And no one listening on youtube is going to notice either as long as the vocals are intelligible without too much bg noise.
A great way to hear the difference, is hook up your mics to your zoom, then output the zoom to the camera, then listen to the audio recorded on the zoom and then the audio on the camera...you should hear a quality difference just in the pres and a/d.
By the way, I'm assuming having proper mic placement to get the best audio from the subject.