Part 2 is now up with what looks like more information on the new HD version of the original game. See link above.
I suppose volume 5 would be DMC4? But what would volume 6 be? I hope not the reboot. Not that I'm hating, but somehow this page seems like the last fort that has to do with the old series and I'd really like it if that would actually be the case.
I know that feel.
And I'm pretty sure the fifth volume is for DMC4 and the sixth one is for the HD collection, finally revealing what the bonus content is alongside a new video.