Beyond that I am fairly confused as to some of the fine details.
There's quite a few fine details, and unfortunately no single source that spells them all out satisfactorily. I've been trying to piece together a picture from a lot of different sources.
There's some sort of i-frame difference between 200mbps and 100 that I don't know enough about to understand.
You're just trading off encoding efficiency & file size for playback performance. In the 200mbps mode each frame is a discrete image which has compression applied internally (intraframe compression) but doesn't depend on information from the frames before or after it - this plays back better when you edit with it and are scrubbing around at random. The 100mbps mode also uses interframe compression, where redundant pixels between frames are only encoded once - this gives you better picture quality at lower data rates, but it's harder on your edit system because displaying a single frame may require up to 15 adjacent frames to be loaded into memory and a lot of calculations to reassemble each frame.
Quality-wise they should be comparable, but that can depend on your subject. Interframe compression tends to fall apart a bit on fast motion, while intraframe struggles more with high picture detail. This may be why they don't offer the 200mbps I-frame only mode for 4k source - the additional detail would likely require a much higher data rate to encode it cleanly. The other side of this is that 100mbps intraframe, which should handle even fast motion cleanly at 1080p, may well look great on relatively static 4k footage but be insufficient for high-motion footage.
VFR info is just marketing descriptions, but the specs details don't seem to list them. One thing I do think might be goood is that it has a true 24p, potentially with a true 180deg shutter (24hz mode for "cinema" frame rate).
EOSHD has a chart midway down their preview that details the VFR capabilities:
http://www.eoshd.com/content/11934/panasonic-gh4-preview
Basically it's got up to 13 frame rates you can choose (depending on your base frame rate), ranging from 2fps to 96fps. So it's got some nice options for slow motion, but it also may be of interest to action projects because you can do things like undercrank slightly at 22fps to speed up something like a fight scene without it looking like it's on fast forward.
Unfortunately I don't think any of this changes the "Panasonic GH what?" response from an owner/operator perspective, but it has some very interesting potential for a lot of uses.
I'm pretty much out of the business of shooting for anyone else at this point, so that gives me the luxury of choosing a camera based purely on my own preferences, tempered of course by the fact that whatever I buy won't likely be paying for itself any time soon. I'd been planning to pick up a bmpcc because I really like the ability to shoot 10bit prores on board such a tiny camera, but it's a trade off for things like a fixed, relatively low-quality screen, lack of a histogram, and no high frame rates. The GH4 is appealing because it addresses most of those concerns, but of course at the trade off of codec quality. Fortunately I don't have much planned to shoot for the next couple months, so I'll probably wait and see what else becomes available at NAB before making a decision.
During the course of the discussion, Illya mentions that the camera has "12 stops of DR":
I think all the cameras have "12 stops of DR" now
