I'm intrigued by the idea of building a G4. I just read this review, however:
http://www.backcountry.net/arch/pct/0008/msg00136.html
and I gotta say that most of the feedback I saw from that guy echoes what I'd expect to be the drawbacks of such a pack. Throwing 1 pound at this bad boy sure would go a long way! I'm thinking about building my own, but at this point I'd consider making the following changes:

1) Closed cell foam pads in the belt and shoulder straps. I'd keep them removable, but I doubt I'd ever bother. We're talking a few grams here...

2) Compression straps. It would be great to be able to keep the load contained in a controlled manner. Even if they ran against the main body of the pack, inside of the mesh pockets, it'd be better to have something rather than nothing. Two would be nice.

3) Adjustable straps and latches for the boat bag closure. It could then double as a vertical compression strap.

4) Custom ~.05" thick aluminum sheetmetal insert for Z-rest pad. I have an idea of what this would look like... basically a flat sheet with a slight angle bend somewhere in the middle, and a ~1.5" flange bent over at each end, then bent downward about 0.5". This would insert into the Z-rest and provide a blunt support surface at each end for the pack to bear against in order to provide the needed structure. Without any holes punched in it, it weighs 1.2 pounds, and would be far superior to a plastic frame sheet.

A new diversion for me to ponder... LOL. I do mechanical design for a living, so it's only a matter of time until I try to over-think a piece of backpacking equipment. ;-)

-Rodney