@TTM there is no definitive answer to that question so far as I am aware but V02 max adaptations have been linked to time above a certain heart rate (i.e. the more the better). My advice would be to do the full 6x4mins therefore but perhaps have a 10-15 second rest in the middle of the intervals as this would clear the legs of the acidic pain but keep your heart rate up for v02 max adaptations.
Though equally given that you presumably have a strong aerobic base but relatively weak v02 max perhaps it would be best to start with a 4x8 mins @ 105% aerobic capacity then slowly reduce the time and increase the % - this way you can build on your strength. There are literally a thousand ways to skin this proverbial cat however.