There is no right answer -- there is just what works for you. Both the GH4 or the A7R II are very capable (full disclosure, I have a Sony and a Panasonic, but neither of those models). Other things being equal (which they rarely are), a larger sensor will give better low light performance, dynamic range, and resolution. Whether there is a meaningful difference between full frame (Sony A7 series), APS-C (Samsung NX-1), or Micro Four Thirds (Panasonic GH4 or GX8), and whether that difference is due to the sensor size, is a hotly debated point in the photo/video world. Some say that it does matter:
Others say no, it really doesn't:
Both of these are worthwhile videos -- you can actually agree with both. I think the rate of technological change is altering some underlying assumptions on this -- think of it this way -- the Panasonic GH4 has 12.8 stops of dynamic range, which outscores the Canon 5D Mark III, even though the latter has a full frame sensor. But the Canon is about 3 years older -- that is how the technology has advanced, across the board. In the end, "scores" on the internet are not important -- it's whether you have an image you love.
That said, I will affirm (since I have a Panasonic LX100, which is a compact camera) that doing 4k video is FANTASTIC. Even just downscaling the 4k video clips to HD, when you edit them, gives you really beautiful HD. Although DSLRs have advantages over mirrorless (particularly on autofocus, though that is starting to change), there really are no 4K options for DSLRs, unless you go for the Canon 1D C, which is older, hugely more expensive, and is lacking a lot of really nice video features (like focus peaking) that are standard in mirrorless or compact cameras. For doing 4K video, and having interchangeable lenses, and access to a housing, your options are the Panasonic GH4 or the GX8, the Sony A7R II or A7S II, or the Samsung NX1 (Ikelite just came out with a housing for it, but Samsung is rumored to be shutting down its camera manufacturing -- which is a shame because the NX1 is supposed to be a very capable camera). Fuji and Olympus both have 4K capable cameras coming, sometime next year.
Back to the point -- larger sensor perform better, but lenses are at least important to the quality of your image. You have to balance off total system cost (camera, lenses, housing, ports, lights, accessories), performance, and size. Choose wisely, since this is a several thousand dollar expenditure!
Consider this option -- rent a GH4 and some good lenses (say, the Panasonic 12-35mm pro zoom, and the 25mm F1.4 prime), and take it out and play with it for a few days. Shoot video, shoot stills, do it in a variety of situations (bright, dark, indoor, outdoor, on a tripod, handheld, etc...). Then rent the A7R II, with say the 24-70 F4 zoom, and the 55mm F1.8 prime, and do the same thing. Take a look at the pictures and the video clips -- which ones do you like better? Show some to non-photographer friends -- just see what an untrained eye thinks. Show some to photographers and videographers and get their opinions. Evaluate the user interface on each camera -- how fluid is your shooting experience -- can you easily access key settings, and will you be able to do so if that camera is underwater in a housing? Which camera is more fun? (That is actually the most important question!)
If desired, you can repeat with other models (the Panasonic GX8 -- newer model -- more biased towards stills, newer sensor, 4K video but not all the high end video options of the GH4) and the A7S II (fewer but bigger pixels -- greater low light sensitivity and more advanced video specs), etc.... Your options in cameras and lenses are going to grow over time.
There is no right answer, it's just what is right for you.