From knob placement in the video, and seeing the gain on that sound is around 1pm, channel 4 looks like it'll be somewhere between green and blue, gain-wise. Assuming his guitar volume is wide-open of course.
Pure speculation of course but, I'd not be surprised if channel 4 turns out to be that additional gain stage on red applied on green instead of blue, and sharing the eq with green.
Like:
channel 1/green - clean
channel 4/purple? - clean + gain stage
channel 2/blue - lead
channel 3/red - lead + gain stage