I was doing some number crunching and if we win every game, we are pretty much guaranteed the fourth spot. We would end up with 69 points.
Roma has 53 and they play Milan so if they lose one and win the rest, which I doubt they will, they still fall a point short.
Udinese has the toughest schedule by far, playing Napoli, Fiorentina, Lazio, and Milan and need to get 13 points. I don't see them doing it in these six games.
Lazio is the biggest threat imo. They have seven games and their toughest opponents are us, Udinese, and Inter. Considering we beat them, they'd have to get 15 points in 6 games to match us. We'd have to hope they don't score a lot because our goal diff's are very close.
This is all considering we win our games, which include Fiorentina, Lazio, and Napoli.