Don't underestimate the effect of a well designed backpack for your wife. With a shoulder worn photo bag, 5 Kg of equipment is heavy to carry on. With a well designed backpack, one which distributes the weight on the hips, 10 Kg is something one barely feels.
The problem with backpacks is, normally, that one needs to take them off to access the equipment. But that would not be a problem if you wife carries the bag: you can just access the camera on her back.
This does not mean that you can load your wife with tons of equipment (she will have things of her own to fit in the bag), just that her ideal carry weight could fit the H4D and the 28mm lens.
Have a look at the bags in a store selling trekking stuff. Your ideal bag should have a compartment usable for the camera and one for your wife's stuff, a large, comfortable belt which is where all the weight go, and a flexible frame so that the bag is separated from the carrier back for ventilation. The shoulder straps do not carry much weight (the belt does) they are just there to keep the bag balanced. Plan to try a few bags until you find one which fits your wife body. It pays.