Yeah, you'd have to see Arsenal as favourites for fourth. They have 3 games in hand over Man U, and are 4 points behind, but also have a slightly better goal difference. Even a win, a draw and a defeat across those three games in hand would likely see Arsenal move to fourth. I'm skeptical that Man U will be able to maintain that position, it seems like they owe a lot of it to de Gea's inspired form at the moment.
Compared to other teams arsenal does have a great chance to get into position 4, because it has several postponed matches, but even so it is not impossible that arsenal can get out of the top 4 later, because they have postponed matches against big teams like tottenham and also chelsea, when you see Arsenal slightly difficult match against Burnley and Wolves made me not sure if Arsenal could get full points against other teams in the postponed match later.
Arsenal sold some important players without signing replacements, that alone is the first piece of their down fall from finishing top 4, when fatigue comes in and when things start heating up more some of their players might fall into injuries and that will really affect their performance and winning games. Any thing can be possible, they can still try to qualify if they can be consistent in winning against smaller teams.