Just got a hint that my GH4/NinjaBlade tests were discussed!
Yes - it's NOT the Ninja Blade downscaling. The scaling appears on the HDMI output side of the GH4, and I think the GH4 does this scaling extremely well. I wanted a quicker workflow than internally 4k/8bit>downscaling in Premiere and more edit friendly files from start. If you watch my moiré test, you would perhaps still want to record 4k internally and downscaling in post if you want the most moiré-free result, but those tests were setup to absolutely worst-case-scenario. So I think the choice to record 4K internally or to use an external recorder to get those lovely 10 bit ProRes files from start depends on either dealing with a more heavy and not-so-discrete rig setup in recording or dealing with the more heavy post workflow downscaling those 4K H264 clips. They don't run very smooth on my timeline, where as we all now those prores files just play like a joy!
One issue however: It seems that you will only be able to record a true progressive signal if recording mode is set to 24 Hz. I hope this will be fixed in upcoming firmware updates from Atomos. If recording from especially 50 Hz in PAL mode, GH4 always will send an interlaced signal, (and only in AVCHD mode), and no matter how I interpret those files in Premiere, they don't look as nice as 24p. Perhaps others have tested this and got better results?
Best, Jacob.