If we compare with the same field of view (obviously there is that crop on the Canon) the A7sII does not look any better than the 5D IV or the EOS R, it is less detailed and while the filesizes are much smaller the quality does suffer from the heavy compression.
The A7III (or A9) does raise the bar in terms of detail, although it is weird that it
does not maintain a true 16:9 aspect ratio, it squeezes it a little bit (somewhat proving that 6k downsampling is not that easy to implement).
What are you basing the image quality comparison on? You are the first person I've EVER heard say that the 5D mkIV has better quality video than the a7sII.
The Sony has a gapless lens design in front of the sensor and bigger photo diodes. The Canon does not have gapless pixels, and it's sensor uses smaller diodes to accommodate Dual-Pixel AF. This design is dated in comparison, and captures less data per-pixel. Not to mention the huge ISO advantage on top of the additional 2 stops of DR, the Sony is noticeably sharper and more detailed. The codec that Sony uses is also found in their $100,000 broadcast cameras, it is as rock-solid as it gets.
The slight stretching of the frame (only present in 4K@24p on one model, the a7III) is definitely a bug that that Sony needs to fix. Fortunately for me, I have no use for 24p. I shoot 1080@120p or 4K@30p, and deliver to 1080@30p for 99% of what I do (professional sports). Since I'm shooting for broadcast TV, my output is always at 30fps and gets sampled down 720p for HD broadcast. We still deliver in 1080p because most content gets distributed on the web where 1080p is an option, unlike broadcast which is still stuck at 720P.
For my personal stuff, the a7III provides the best quality imaging I can afford for stills and video in one package, with one set of lenses. At work, if I'm shooting to tape I'll always grab the a7III because it is small, and lightweight, allowing me to handle the camera easier at more angles than I can with a heavy broadcast camera. The best part is that the image quality is indistinguishable from our $100,000 ENG cameras, and our editors love it because it is the same codec and color profile they are used to working with already.
I'm really excited that Canon is coming out with new stuff, I started as a Canon shooter originally. As I got more serious about broadcast television, and got more high-end gigs I noticed that in the sports world, Sony dominates for video, and Canon dominates for stills. For people like me, who shoot both stills and video professionally, a camera that forces a crop at your highest-quality settings is a deal-breaker. I often shoot on a long prime, and don't have the time to swap lenses between stills and video. Canon gave the professional crowd a big middle-finger with their implementation of 4K in the 5DkmIV and EOS R.