Flexibility to move on is never a bad thing.
Scenario 1, he puts up 30-40 goals a year on a contender/playoff team (easy extension opportunity with team success. Cap hit will likely be similar to 8-9 mil again)
Scenario 2, he puts up 30-40 goals a year and the team still stinks (flip him for a decent package year 3 or 4)
Scenario 3, he falls below expectations and you get out of a bad contract while you continue to rebuild.
Most unlikely... Scenario 4, he stinks and the team becomes a consistent playoff team/contender.
Not a lot of negative to committing only 4 years.