I did do a lot of video before priorities forced me to concentrate on stills, but with the GH1/2/3 rather than a Sony. Video was what brought me into m4/3 to start with, and that system was my choice for several reasons:
- Small size, but big batteries, particularly on the GH3/4
- Fantastic ergonomics with the GH3/4
- Very good build quality with the GH3/4
- Many lenses to choose from and even more that can be adapted with or without a speed booster
- More depth of field (too little DOF is more of a challenge with video than with stills)
The A7s is a nice video camera, particularly in low light, but it's no secret that there are things about it that I don't like (so take what I write here with a pinch of salt). Panasonic has shown that they are very serious about video, and the system is very flexible. Add to that all the excellent lenses, including the Voigtlander f/0.95 primes, and this is the system that I would go back to if I was to get more serious about video again.
The alternatives would be the new Blackmagic Micro cameras, which are cheap, totally modular and extremely flexible. However, when all bits and pieces are added, they won't be cheap anymore. The GH4 is the do-it-all solution.
However, the biggest challenge shooting video is not the camera, but the different way of thinking, the complicated post processing, sound, movement etc. Video is not stills that move, and anybody thinking so will struggle to get it right. So my recommendation would be: Keep your A7s, use it with as few lenses as possible, make storyboards before you shoot and spend time in front of the computer, experimenting with post processing. There isn't really a video equivalent of a snapshot. Even the simplest sequence requires a plan; a start, some content and a finish.