I *DID* get both and by far I use the A7S more. I will explain my progression of thought on it as my experience evolved and hopefully it will provide some insight. Originally I bought the GH4 because I wanted the in-camera 4K. The 4K is nice and so are the rest of the features in it but I still had the same old problems in low light, the constant bane of my existence. That said, the feature set is extensive and it's a very nice camera. Form factor is pleasant (smaller) coming from a 70D. The only reason I didn't get the A7S outright from the start was because, like you, I was 4K hungry for a decent camera and didn't want to shell out for the external recorder. I had already purchased the Blackmagic Production Camera 4K and was sorely disappointed by the Maximum 800 ISO and the ergonomics of a brick along with an embarrasing number of other disappointments. I lamented the challenges of using the Canon glass from my 70D on the GH4 and even though the MFT sensors these days are amazing, I wasn't crazy about getting a smaller sensor than my existing 70D which I enjoyed quite a bit. I couldn't really justify a new camera, especially with a smaller sensor unless it had 4K. The GH4 I'm told has a very easy to understand interface, which I mostly agree with, except for the fact that the menus are overwhelming. Some tabs have 9 pages of options and finding the feature you want is a bit of needle-in-haystack. And then when you find it, it might be disabled because of some weird dependency on another feature setting, triggering off the hunt once again. I didn't like the proposition of having to buy glass all over again for MFT, and although I had already bought some decent glass for it, would have liked to use my fast 70-200, and other lenses without having to buy new all over again. I got the Panasonic 12-25 (24-70 equivalent) and the Panasonic 25 (50mm equivalent) f1.4. I read some reviews on the f1.4 versus the olympus f1.8 and the image was much better and the bokeh negligible, so I took back the Panasonic lens and got the Olympus, which is far superior, even if it doesn't have stabilization. There's a problem picking good glass for MFT. A lot of the good MFT lenses don't have stabilization (Oly lenses take advantage of the body stabilization in Olympus cameras), and the GH4 doesn't have it in-body. Another catch 22. I saw some reviews on the A7S from Philip Bloom and Dave Dugdale and had a restless night's sleep, waking in a cold sweat to jump in front of my browser and research some more. It turns out that Metabones has a smart adapter for the A7S with full electronics, including (slow and and unreliable) AF. I was used to AF being a manual proposition, but knew that I actually COULD use my Canon glass, which I already use on my Blackmagic. I dreaded the constant challenge of always balancing catch-22 situations and finally brainstormed a solution that didn't have to involve major compromises on every single decision: I could get the A7S and the metabones smart adapter to use my Canon Glass, and then bite the bullet with the purchase of an Atomos Shogun for the 4K after all, and enjoy the benefits on a beautiful display, RAW recording, XLR phantom power breakout cables, etc. Because I already had a Blackmagic 4K, I realized I could also use SDI to output and record externally on the Atomos with that, and not have to fight its awkwardly dim and poorly placed sensor. The Shogun even has scopes and focus peaking, which takes the need for using that feature away from the brick and puts it on a much nicer display that I can move around. Heck, it might even keep the Blackmagic from burning my hand running hot if I externalized the recording. And THEN I read that the GH4 outputs 10bit 4:2:2 from it's HDMI port. That meant that instead of having to suck it up and play with H.264 from the GH4's internal recorder, I could actually do better with the Shogun and record RAW (or 10 bit prores HQ) using the Shogun and save time in post with files that are already much better suited for editing. I had previously been looking for a good screen to augment the one on my Blackmagic and maybe use on the 70D from time to time, but the value on the GH4 and the Sony added to it made it a no-brainer. No longer was the Shogun an unwanted luxury, but an affordable option I had forgotten I'd actually been seeking already. Not only would it solve the 4K on the A7S problem but actually made the GH4 output better with 10 bit 4:2:2 and made my Blackmagic no longer the unwanted step-child. I went to the store and grabbed the A7S and native 35mm lens for starters. I then grabbed the Metabones smart adapter and started to play with my Canon lenses on it. After that it was time to put a deposit on the Shogun. I began shooting at a venue in town that I had otherwise somewhat shunned. It was a really cool place with great atmosphere and honestly, the stage lighting was pleasing to the eye. Unfortunately, it was just shy of what you would need for a camera. I pulled out the A7S and it made what used to be a nightmare into a dream of luxury proportions and lurid colour. No longer was I fighting my gear, but instead I was freed from the fight, and just enjoying composition once again. I had been broken of the stockholm syndrome of blaming the venue for "bad lighting" and got to embrace the nice setup they had, which had always been more than adequate for naked human eyes. I went for a walk outside at night and shot the street like it was daytime. Light was no longer an obstacle. Instead it was a choice. I could pick a fast shutter, tight DOF, deep focus, whatever I wanted. It's true, you can shoot in low light with any camera, provided your exposure is adequate, but not video, and not hand-held. I went to another venue with "bad" lighting and enjoyed the high of random shooting at 20,000 ISO just for fun, and ran circles around the other photographers trying to get their AF to catch and edge on something, anything. I felt guilty that I was using the A7S so much and not the otherwise brand new GH4, so I took them both out to that low light venue. GH4 is supposed to be pretty decent for it's kind, and I thought I would properly field test it in the circumstances I wanted to be able to shoot in. The results were interesting to say the least. The A7S made shooting the venue effortless, getting tons of light from the stage and pristine footage. The GH4 on the other hand had significant blocky artifacts through all the footage. I manually set the GH4 to 1/60 shutter at 3200ISO with aperture wide open at f2.8, the best I could get away with. If I hadn't cleaned it in post with NEAT video, it would have been unusable. What I learned was profound. A more sensitive sensor will change the way you think about shooting. I also learned not to clutch so tightly to my old lenses and habits of compromise, the "stockholm syndrome" I described. Instead I finally found a camera that I didn't have to fight with on location OR in post. It's like a breath of fresh air while having shackles cut away from your wrists. I'm attaching stiil frames from the GH4/A7S two camera shoot I described so you can see for yourself the difference. This was shot at 1080p, so it doesn't demonstrate the 4K capability, just low light, as my Shogun has not yet arrived, but you will see how drastic a difference it makes to have some freedom in exposure.