Happy to share any useful info. Like you, I was looking everywhere for people's experience with the 5D4. There's a very talented Cinematographer/DOP who has several music/model videos on YouTube, where he uses the 1Dc and 5D4 cut together. I found it difficult to tell them apart in the final edit. And in the comments section, people asked which he preferred or would buy if he had to make a choice and his response was to go with the 5D4....
As for my own experience.....I realized that I needed a hybrid. Was going to go with the Sony Full Frame until footage was released online. I can't get past the shitty skin tones and the ridiculous noise reduction=mudd. And I'm just not in the position or place in my career to warrant a dedicated video cam. Otherwise, I would probably go for the C200 or something like that. And I know I sound like a Canon fan boy but their image and usability on the day can't be beat. For my film I had originally rented a GH5.....a few hours later, it was back at the rental house. It was like a toy with a confusing menu.
My point being, I need gear that works, out of the box and intuitive in its use. I'll use whichever tool that allows me to get the shot or footage without getting in the way. And so far, thats been Canon.
On set the 5D4 was amazing. Never once an issue. I used the amazing DPAF coupled with the Canon app on an iPad to frame and focus the shot. The app worked accurately 80% of the time and was truly a life saver. I would have wanted to send the body back to Canon to upgrade to C-Log but just didn't have time in the shooting schedule. Used Neutral Profile and with VERY limited lighting, was able to pull out amazing dynamic range and IQ. The 4K is true Cinema DCI and coupled with the crop, is a very pleasing, cinematic frame. No need for adding 'black bars' to the edit.
File size.....yep, they're huge. The film is just shy of 11 minutes. Rendered and exported in ProRes 422 HQ, with the applied LUT's, grain and audio; came to 63 Gigs in FCPX, 53 on the external drive and 49 going into Vimeo. But the quality! Wow! Seriously, everything from retaining dynamic range to the colour in the most organic way. Totally worth the upload time!
Things to consider....if you want C-Log, you need to either send away for installation or buy pre installed from BH?
Must have big, fast CF Cards. The camera will split clips into 4Gig sections if the care is smaller then 256Gig. In 4K thats about a minute of footage. Did I mention needing a lot of cards? But I bet its still less then shooting RAW on BMCC or RED....or having to buy the really expensive CFast or the SD cards to be able to do 4K in the GH5 or the Sony's.
When using the Canon App with the 5D4, you have to turn on the wi-fi in camera, which uses even more battery then shooting video. Haven't tested for length of use but easily used 3-4 in a full day of shooting.
I have never used the C100 but the footage is nice, without the limitations of a DSLR/Hybrid. Its probably super user friendly and gets the job done and if you don't need 4K, they're very reasonably priced.
That's all I can think of for now. If there's anything specific you'd like to know, ask away. Happy to help.