Short, and surprisingly uninspired story. It was good, but it wasn't infamous 2 standard. Most reviewers seem to agree.
However, the action is awesome, and going around the city is a blast. Overall its a great game in my opinion.
But, you're right, especially for a PS4 game, I expected it to be as long if not longer than infamous 2. I do have a lot of extra dlc stuff to do though, like cole's legacy.