Our Viper 4, minus the reservoir which comes with it, weighs around 420 grammes and is a simple beast. The back system uses a flexible plate coupled with a thin layer of mesh-covered foam and the reservoir compartment is under pressure, but the tensioning arrangement is simpler with a press-stud closure rather than two snap-buckles.
There’s a small zipped pocket for valuables but everything else has to share space with the reservoir. You can use a 3-litre one, we do, but that reduces storage for other items. There’s extra storage from a shock-corded accessory patch on the back of the pack though and the series all come with Osprey’s Lid Lock, a neat plate that goes through a helmet vent and holds it in place. There’s a little gel pocket on the shoulder strap too.