The end of your life is not guaranteed. Your life may be shortened any number of ways unrelated to how much sleep you have had.

Let's say you live to be 90, sleeping 8 hours a day. That's 60 years of actual living time, 20 of which occur between ages 60 and 90. If you sleep 4 hours a day and live to be 90, that's 75 years awake with those extra 15 years spread over your entire life - not just the last third which is likely to be less enjoyable and productive.