Cheers HD.
I think mine is the first G4 to be sent out into the wild. Not sure how long Bill has been working on the G4. I got lucky that when my G3 build came up, Bill offered me the opportunity to have the G4 built instead. He sent me many vids during the build, but I'm not at liberty to share them. I believe Bill will post something official soon as he's very excited with how it's turned out.
From what I understand, the main revision is a different diode or set of diodes that Bill has come across. I also had Bill remove the Reverb control and move the Depth control to the front panel, and add Bias Adjust Controls to the rear panel.
For reference, I owned a G3 with the Pure Clean channel a while back and although I didn't bond with the Pure Clean channel, I loved the gain channels. I can hear a difference in the videos of the G4. It's mostly noticeable on single-note soloing. There's more of that wet, sticky, saggy sweetness that the great JCM800's had, and with really nice note clarity. It's hard to describe. Kind of more saggy and elastic sounding for leads, but still plenty tight when playing chords or chugging (sorry for the stupid tone descriptions; I'll post a vid when it gets here). In short, the new revisions sweeten the voicing of the overdrive.