For years, I have used the kdenlive free video editing software. When you look at the list of video resolutions to choose from, it may seem that the highest resolution you can edit is 1920x1080.
However, it is a simple thing to add 4K resolution to the list of formats to choose from. Then, you can edit 4K video just as easy as 1080p. Here's how:
First, go to the "Manage Project Profiles", to add 4K by clicking "Create new profile":
In this new profile, you fill in a name with "4K" in it, and the resolution 3840x2160, set the frame rate to whatever you are using, e.g., 25 (typically used in Europe), and 30 (typically used in the US):
If you use the Cinema4K resolution, you should set the resolution to 4096x2160, and the frame rate to 23.976.
You can then edit the Project Settings, and select your newly created 4K profile. Then, import the 4K video files and edit them as usual.
When rendering your edited movie, you do it in the normal way. In the example below, I used a video bitrate of 60000 and the H.264 codec:
This movie was recorded using the Lumix GH4 and the Lumix G 100-300mm f/4-5.6 (my review) and Lumix G 14-140mm f/3.5-5.6 (my review). I edited it with kdenlive, as described above.
Rendering the movie, of about three minutes, took around 2 hours on a five years old PC running Ubuntu Linux.
I generally set the shutter speed to 1/50s, to achieve a 180 degree shutter at a frame rate of 25 fps. Since I did not use any ND filters, this means that the aperture was often stopped down to around f/8-f/16. I had autofocus enabled all the time.
Some comments to the video.
At 0:15 and 1:02, I am zooming in using the Lumix G 14-140mm f/3.5-5.6. While the GH4 certainly does autofocus very well, the AF is not as efficient while recording 4K video. You can see that it loses focus a bit while zooming, but it is not a big deal. Normally, one would not want to zoom excessively during video recording.
At 2:00, I am using the Lumix G 100-300mm f/4-5.6 at 300mm. This produces some artefacts. First, there is a reduction of contrast, often seen with long lenses. Using the supplied hood probably helps a bit, but ideally one would want an even longer hood when using the lens in the longest setting.
Second, you will see a lot of wobbling in the video. This is not because of the camera or the lens, but it is due to atmospheric distortions. The light travels through a lot of hot air before reaching the camera, and that causes the wobbling effect. There are no solutions to this, except, possibly, to use a shorter lens and going closer to the subject.
Using the Lumix G 100-300mm f/4-5.6 at 300mm, I found that the GH4 would some times fail to focus, even in bright daylight. It would become stuck in the minimum focus range, and I had to manually focus it to infinity. This behaviour surprised me a lot, as the GH4 is otherwise very good at autofocusing.
Another thing to note is that the video is more prone to rolling shutter effects in 4K mode, than while recording 1080p footage. This is because the camera reads out the 4K footage slower, taking about 1/30s to read each frame sequentially. With 1080p, on the other hand, one frame is scanned in about 1/90s, reducing the risk of rolling shutter artefacts. To avoid this problem, using a tripod helps a lot.
Apart from this, I think the Lumix GH4 does 4K video very well in this example video.