Quick (yet complex question) as I'll be getting the camera right before I head out on Vacation.

Moving up from a GH2 here and going to get the GH4 on Tuesday. I'm looking to film either UHD or C4K and downscale to 1080p for my films [while uploading 4K stock to Pond5].

I'm wondering which is better as my initial guess is to film C4K at 24.00Hz Internally - Cine D; for a note I'm using Premier CC and Red Giant to Edit/Grade - eventually I'll look towards Resolve but for now 'Looks' and 'Mojo' do what I want when I get a nice flat image.

Also Should I be filming MOV or AVCHD? On the GH2 it was AVCHD but that is a pain in mainly ways compared to MOV and if both are equally good I'll swap over to MOV.

Your first question I can't answer but the second I can. AVCHD or MOV don't make a difference except in usability if the file bandwidth rates are the same. It's just a file wrapper change that helps when using the camera files in a QuickTime based application.

There is a huge difference between MOV and AVCHD, but it's primarily bitrate based. AVCHD tops out at 28Mbps while the MOV format starts at 50. So it depends on your desire between balancing file size and robustness of codec. You will also not get some of the shooting modes if you pick AVCHD,

As for C4K, no reason to shoot in that mode. You will merely add letterboxing to whatever video you shoot as C4K is slightly wider than QHD. Plus you get 24.00 instead of 23.976 as a framerate, which is geared towards film out.

My mistake, MP4 can do the same bit rates as AVCHD as set up in the GH4 as well as higher bit rates. MOV is limited to 50Mbps and higher. All of these are using the same h264 codec.

Then I should use MP4 then, Assuming i don`t need TimeCode from AVCHD

The more I read the more I lean towards UHD and 23.98fps as being more compatible with downscale to 1080p, especially when you consider that currently none of my other cameras currently do 4k @24.00fps (specifically the 24fps vs. 23.89 which would need an adjustment in post every time I include a clip).

I'll switch to 24FPS later when I start needing more DCP output files.