Invulnerability on a Sentinel: Durability not available at level 4 during respec

[IronYeti]

I was working on a respec of an AR/inv sentinel yesterday. After selecting Disorienting Shot at level 1 and Buckshot at level 2, Durability was not available as a level 4 power. Usually at 4th level, T3s of both primary and secondary are available, right? So, this seems like a bug.