There was never any question about it happening. The option is part of the rookie wage scale that requires first round draft picks to sign four-year contracts and allows teams to pick up a fifth year.
Castonzo did an excellent job protecting quarterback Andrew Luck's blindside last season. There's no reason to believe he won't continue to do well. He is very durable -- only missing four games with an ankle injury during his rookie season -- and he and right tackle Gosder Cherilus are the pillars on the edges for the Colts.
Castonzo only gave up four sacks last season. The four sacks given up were almost 10 fewer than Tampa Bay's Donald Penn (13.5 sacks) and almost eight fewer than Bryant McKinnie (11.5 sacks) gave up last season. There were 46 tackles who gave up more sacks than Castonzo last season.