Well, 4 bar is about 133 feet of head. I'm guessing that he doesn't have a 100 foot waterfall on his property, he has a stream that loses 100 feet of elevation over some large distance.
If you put a 4 foot dam up and block the stream, you have 4 feet of water delivering pressure. If you use a pipe, you can have lots of feet of water delivering pressure (1 bar is about 33 feet of water, so this installation is at least 60 feet of water delivering pressure).
The reservoir provides an opportunity to even out the energy production (and helps with flood prevention and irrigation, two big motivators of dam building).
The other factor is that the power achieved by the station will roughly go with the volume of water and head. So if you want a lot of power you end up needing huge pipes.
Basically In hydro power to generate power you can go for a small amount of water falling a long distance or a large amount of water falling a small distance
Its a classic high head low flow pelton wheel installation.
He wouldn't need such large pipe for a penstock if he was using a pelton wheel, but he also doesn't have the head to drive a pelton efficiently - the more the better for one of those.
I've seen systems that put 2kw of energy through 2" pipe! 350+ feet of vertical head will do that.
Yeah it's basically a modification of a pelton wheel, sort of adapted for higher flow and lower pressure. Technically a turgo wheel IS a pelton wheel, but they serve sort of separate purposes.
