Doing for the first time the JVC 4K workflow in Premiere Pro (Windows 7 64 bits) (click for a larger picture) :
No conversion needed on PC to get a 4K stream.
Just put the 4 HD streams in the timeline. Set them at these positions :
SD1 960 540
SD2 2880 540
SD3 960 1620
SD4 2880 1620...