I was a little surprised by this, so I went and grabbed and weighed the unopened one I have waiting to replace the one that is starting to show its age. It is indeed 4.5 pounds and the old Lat64 bag I replaced it with was only 2 pounds.
I was really surprised, because my recollection was that the BP1v3 felt way lighter than the Lat64 bag when I started using it. I'm going to guess this is simply down to how the bags are constructed and where the weight sits. The BP1v3 uses the design of the straps, rigid panels, and the back padding to keep the weight from solely hanging from the shoulders.
Basically, I think I'm just agreeing with jupiterboy and Keith H that the design of the bag is much more important than the specific weight, because what's in the bag is going to weigh much more than the bag itself.