I know a lot of conditioning experts from various sports who say that training longer than the actually contest would be is not good. So in this case, a 4min round isn't as good as a 3min round becasue the brain will respond and subconsciously you'll adjust your intensity to go the extra minute.

if i told you to run 100m, you'd go flat out.
if i told you to run 200m, you'de ease up just a bit so as not to gas.
but if i told you to run 100m and it was actually 200m, you'd go harder than if i had told you it was 200m.

it's basic human survival instinct.

Intensity is key for stamina. Even marathoners now know the values of interval training and sprints.