Yup, a weak 4. But he has the potential to be really really good - but never a defensive powerhouse like Abi or Chiellini, but he has other qualities they don't have.
Besides his defensive work he needs to improve his passing. In the Euros and Valencia I've seen him give away the ball far to easy - in the spanish NT they play with 2 DM's and Arbeola defensively when Alba attacks. In Barcelona when we have pushed up the whole team, with usually only 2 CB's at the back (even Busi is far up) we will be very exposed.