What’s important here are the absolute gains not the relative ones: The sessions were done at the maximium tolerable intensity for the prescribed intervals so adding more to make up for the shortfall of the shorter intervals would have been impossible.
8 x 4 minute intervals would have to be carried out at a much reduced intensity, very close to that of the 4 x 8 minutes intervals but the additional rest period would reduce the amount of time at >90%VO2max.
Mike