#4 fullback is a position that will turn over often. Barca needs someone for this role who is too good to play that rarely.
Alba will be with Barca for a long time. With his incredible versatility, Adriano is one of the most valuable non-starters in the world. I'm sure even Adriano realizes this is the best role for him, given his propensity to injury. Barca does not re-sign many players over 30, especially those with physical problems like tendonitis, so there are valid reasons to let Alves go after this season. On the other hand, even if he might not be as good as he was a couple of years ago, Alves is fundamental to the Barca style of play. And he is Messi's buddy, and Neymar's buddy. If I were Montoya, I would only re-sign if Alves leaves. Otherwise, a player Montoya's age has to seek more playing time, even if it means going to a lessor team.