research has shown that the longer work in one day the lower your performance gets. meaning that eg in the first 4 hours i get 60% of my work done and in the remaining 4 hours 40% simply because i get tired later in the day. (numbers made up for illustration purposes only)

4 hours of pair programming per day can therefore possibly be enough to make up for the increased loss of performance in the rest of the day.

The world is full of examples of companies that reward people who are more productive in a few hours by letting them go home instead of raising expectations and working them to death, I assume.