Of course, the big news is that it supports 4K recording (and both QuadHD 3840x2160, as well as DCI 4K 4096x2160.) Just as importantly, while internal recording is limited to 4:2:0, you can output 4:2:2 in either 8-bit or 10-bit. You'll have to buy the base module (DMW-YAGH ~$1,000?) and a (currently) expensive third-party external recorder to do it, but you can do it.
Also interesting is that external box that Panasonic has produced for outputting 4K video, as well as adding XLR audio inputs. (it's the box at the bottom of the camera in the picture above.) It looks big; though remember that the camera itself isn't that big to begin with. This box really suggests that Panasonic sees this as a serious video camera.
Panasonic has done well with their GH3 cameras; a lot of people have praised their video capability and while it never got the buzz of the Canon 5D Mark III, I know a few documentary filmmakers who have used the GH3 for their projects. The GH4 could be the low-budget camera for independent filmmakers this year.
Andrew Reid offers a lot of information on the camera, including notes about the way the sensor is cropped. He thinks that's a bad thing, and if you're dealing with lenses, then yes it is, but if you're trying to avoid moire in your images, cropping is probably a better option than having the camera doing some fast resizing of the image:
Hands-on preview of the powerful 4K shooting Panasonic GH4! | EOSHD
When 10bit is selected the encoder is bypassed altogether and you can record extremely nice 1080p in ProRes 10bit 4:2:2 on your external recorder from an actual 10bit 4:2:2 source signal. The camera cannot record to the SD card whilst set to 10bit over HDMI
The 4K Panasonic GH4 is here. You can even get 4K SDI out of this thing! | Red Shark News
What does distinguish the release, though, is all these images of the videography base station (which the company calls DMW-YAGH), a plug-in, or possibly sit-on, module with proper connectors, XLRs and BNCs for sound and picture, and manual audio level and phantom power controls.
Specs and info:
Panasonic Lumix DMC-GH4 Mirrorless Micro Four Thirds Digital Camera (Body Only) | B & H Photovideo
Panasonic GH4 – shoots 4K, 10bit via XLR/SDI unit, mind: blown | Cinema 5D
Unfortunately, all the 4K varieties are 100MBps only (IPB frame order), while you can record up to 200MBps (ALL-Intra frames) in every 1080p Full HD variety. Full HD video can be recorded in up to 96 fps (at 24p playback), which should be great for some amazing 4 x slow motion footage.
Lots of pictures of a prototype camera:
With 4K GH4, Panasonic Comes Out Swinging | Filmmaker Magazine
Per Panasonic, this working prototype is merely “70%” a finished product! I stress this point, because the audio controls on the DMW-YAGH Interface Unit are obviously not in finished form.
4K Panasonic GH4 Has Arrived: 4K Up to 24FPS, UHD Up to 30FPS, & 1080P Variable to 96FPS | No Film School
Pricing will be announced in March (but less than $2,000), and the 4K mode is a slight crop (from 2x to around 2.3x as compared to full-frame). The UHD crop is only slightly more.
This seems like quibbling to me:
Panasonic Brings 4K to Mirrorless Cameras | mashable
In its speed to get a 4K recorder out, Panasonic may have short-sheeted the 4K professional market for which the GH4 is intended. The GH4 will record full 4K — but "only" in 30p. Full 4K is actually 60p.The GH4 also lacks 60p 4K output-compatible HDMI 2.0 output and HEVC/H.265 encoding, which is about 40 percent more efficient than current AVC/H.264 compression.