Hey
1MOREMOD - ok, lets think through this in a linear fashion....
2 different people serve as POTUS.
1st person has the first 4 years, then the 2nd person takes the next four years.
It does not matter that they repeat this pattern, or that the 2nd person could hold the office for 8 years...
The math is simple - could be 1 year between POTUS at the end of the term (like we have now), or the current POTUS term where he is serving 2 consecutive 4 year terms, hence the 8 years.