You stress different energy systems with 8 and 4 reps.
I think the total volume thing that's going around is a bit too strong of an oversimplification. For example, look at an Olympic weightlifting program and a bodybuilding program. Both have high volume (overall). But they accumulate volume in totally different ways and the lifter's bodies reflect that.
If that anecdotal tale isn't good enough, metabolic studies of muscles show that lactic acid accumulates linearly over the course of the set. Ammonia, on the other hand, shows a biphasic curve where it is initially steady up until some point in the set where it decides to move upwards. Interestingly, this point is right around where Prilepin says it is optimal to stop the set. In practice, preventing the rise of ammonia means you need less recovery time between workouts, since there is less mechanical damage to the muscles. That lets you train Olympic style. With bodybuilding style you're going to go way past that point and need a bit more recovery time. That's not to say both groups don't venture to the other respective side once in a while. All successful programs, that I am aware of, mix a little bit of both at times.
One could take that last statement I made and say "when you look at the big picture volume is the only thing that matters". But I think that misses some of the details. And it is possible to intelligently manage those details.
