I have them all. I have tracked calories burned & mileage done with each. While Vol 5 is longer than Vol 4, I have found that minute to minute comparison, Vol 4 burns more calories faster than Vol 5.
To give you an exact break down of what I'm talking about...
Vol 4 is something like 45.5 minutes long. Doing all the challenges at my perceived levels of exersion I burned 214 calories.
At the 45 minute mark in Vol 5 I only burned 160 calories using the same speeds, exersion levels & tension settings. In my opinion, that is HUGE.
I can't work out for an hour + most days, so if I'm going to reach for a workout that I can cut short but get MAX caloric burn in minimal time, I'm going to reach for Vol 4.
Others may or may not have had similar experiences. That being said, the workouts are all very different from each other but my all time fav is Vol 4. You can't go wrong with whatever your choice.