Just played around with the 4K timelapse functionality and writing this just in the hope that someone else might find it useful
Basically the 4K timelapse (that you can activate in movie mode by pressing the "Up" 4-way controller and selecting "Interval Movie Record") records it's output in an AVI file with MJPG (Motion-JPEG) compression. I did a 15 min recording with picture taken after every 2 seconds.
That should give me 15min * 60 (sec/min) / 2 = 450 frame video.
What I actually got was a 1353 frame video that was at 24fps and about 56 seconds long.
Since this was somewhat unexpected I investigated a bit and I can clearly see that each and every frame was in the output file 3 times.
There is no difference whatsoever between those frames.
MJPG is not like MP4 or other common compression methods that compare frames to the previous one and can compress identical frames a lot so sadly this means the file is actually 3x larger than it has to be. In reality the output is a 8fps video with frames duplicated twice to look like 24fps but it gives absolutely no advantage.
It's fairly easy to get rid of those duplicate frames completely (and without any loss of quality) by using for example VirtualDub and setting "Direct Stream Copy" and "Process every 3rd frame".
Pity though that such a waste of time, card space etc. was deemed necessary by someone.
My personal guess why it was done like that:
Likely 24fps was chosen for the timelapse as the lowest commonly used standard framerate (lowest is better as we all want our pretty timelapses to last a long time) - and then likely someone higher up the corporate ladder complained that their resulting timelapse was too short
I know it sounds stupid but it's such a stupid "feature" - ok, bug as I wrote in the title already - that I just can't think of a better reason.
Hope it might be helpful for someone else who uses this feature.
And I know that "pros" just shoot pictures and convert them to video on the computer, not in-camera because that gives more creative freedom etc.