Perhaps in low-light, but the A7s has major problems outside of it's realm as a pretty specialized camera. There is a lot you have to do to it to baby the image, and that 3200 native ISO provides ceaseless headaches for anyone that wants to shoot anything during the day.
For under 2.5K, you have the Black Magic Pocket at less than a third of that price, you have the Black Magic Cinema Camera at $500 less, you have the Nikon D750 (WITH Atmos Recorder!) for the same price, the Samsung NX1 for $1000 less, Any of the Rebels with Magic Lantern (at between $2000 and $1200 less), The Panasonic GH3 and GH2 (Hacked) (At $1800 and $2000 less respectively), etc.
For $500-800 more you can get a Digital Bolex, a used Red MX, a 5DMKIII, a Nikon D800, a Black Magic 4K Production Camera, all capable of shooting at higher than HD, or Raw (with a recorder for the Nikon, which is what they shoot "Wilfred" on), and internal with Magic Lantern.
And that is simply not true, the GH4 sensor is almost twice the size of 16mm film. So the claim that is "doesn't look cinematic" well... "Lock, Stock, & Two Smoking Barrels", "Station Agent", "The Decalogue Films", "Bergman's Magic Flute", "The Life Aquatic With Steve Zissou", "The Squid & The Whale", "Leaving Las Vegas", "The Walking Dead", "The Hurt Locker", and "Midnight Cowboy" would all like to have words with you.
Also the S-Log still has major problems, with it's baseline ISO of 1600, and with the A7s it's even worse with having to have a baseline of 3200iso. Either way, S-Log is more of a novelty outside of low-light circumstances, you're going to have to stack so much ND and IR on the front of it, that it's going to be very difficult to shoot day externals with S-Log on either.
Not to mention the A7MKII is still $200 more expensive than the GH4.
So you can afford a GH4 and a Speedbooster, and get 4K out of the box, or you can get a A7s that doesn't have internal 4K, but shoots better in low-light.
The Sony's are great cameras, with a lot of problems, but the idea that it looks "more cinematic" because of it's sensor size is verifiably false.
There have been some truly amazing films shot on smaller sensors and smaller filmstocks, the cinematic look has little to do with sensor size.