4K was obviously the big marketing buzzword in 2014, but in all honesty most of these "4K" cameras actually produce quite poor 4K video. Take a look at a 1080p crop and the quality is much worse than any decent 1080p camera from 2013. Pixel for pixel, the 2.5K Blackmagic Cinema Camera or GH3 or C100 are just as good, if not better. It's when the 4K image is downscaled to 1080p that they really shine as good cameras.
- GH4 - Internal 4K recording at 100Mbps produces small file sizes and grades pretty well. Footage is sharp, especially when downscaled to 1080p. Tons menu options to tweak video settings. Cinema 4K (4096 x 2160 resolution).
- a7s- Perhaps the best performer ever in super low light. Excellent dynamic range.
- Blackmagic Production Camera - Prores and RAW 4K recording. Global shutter.
- GH4 - Produces a somewhat "video-ish" look. Suffers quite a bit from micro-jitters. Small sensor/needs a Speedbooster to really shine.
- a7s - SLOG is difficult to grade and its native ISO is 3200. 4K only with external recorder. Worst new camera for rolling shutter. With Atomos Shogun, it's by far the most expensive of these three cameras.
- Blackmagic Production Camera - Poor low light performance. Dynamic range is not as good as other cameras. Extremely low battery life/requires an external battery solution. Problem with fixed-pattern vertical noise bands even at low ISO. Huge file sizes.
Older Cameras that may still be better options
- Canon 1DC - Produces perhaps the most cinematic image for under $10,000 with its Cinema 4K resolution 500Mbps Motion JPEG. It does suffer a bit from banding and no focus-peaking could be the deal-breaker, but overall this could be the best camera overall for under $10K that's still in a DSLR package.
- Canon C100 - Sure, the C100 Mark II is technically a new camera, it's basically just the old Mark I with a new image processor and a better screen and viewfinder. For the price the C100 might just hit the sweet spot of ergonomics, usability, and image quality.
- Blackmagic 2.5K - If it were not for the issues with battery life, poor functionality, and poor form factor, this camera could still be the top dog. With it's excellent dynamic range it produces a cinematic image on par with some of the best cameras in the industry, even at higher price points. If Blackmagic re-released the internals of this camera in a C100 or GH4 style package with a few new firmware features like auto white balance, this would be a real winner in my book.