Per ShamSports, only the first three years (and around $16M) are guaranteed.
The fourth year ($5.845M) in unguaranteed, becoming fully guaranteed if either:
(1) Webster is not waived before 7/1/16
(2) He plays in 180 or more games in the previous three seasons
So the Wizards have a measure of protection in the event of a major injury or repeated injuries that keep Webster out of more than an average of 22 games per year.