Downsampling 4K footage for an increase in chroma sub-sampling
As I understand it, ffmpeg creates a raw image buffer with the input files it is given, then applies effects such as scaling and then encodes. So if you use ffmpeg to do the scaling and encoding to a 4:4:4 codec it should do what you want:

ffmpeg -i "my_gh4_422_4k.mov" -vf scale=1920:1080 -c:v prores -profile:v 4 -c:a copy "my_prores_444_1080.mov"


It would be interesting to test this, but I don't have any 4:2:2 4k footage on hand. I believe ffmpeg also does dnxhd 444, but I can't find any documentation on how to specify it.