I don't think there's much chance of Man U taking fourth spot, I think the top four is pretty much set now. Man U just don't have the consistency, and their recent results aren't as strong. I think their season is effectively over, all they have left is to hope they can finish in fifth or sixth and don't slip further down the table.
Arsenal's game in hand means more now that there are fewer games left in total.
No doubt with Manchester United could finish on 4th position place because several match left against with Arsenal, Liverpool and Chelsea where looking on statistic not any lucky for Manchester United. I think left several match for top position standing team I see Tottenham Hotspur have potential back to 4th standing place depending left match on Primer League just with bottom position team, not any top four team standing against with Tottenham Hotspur and most lucky if success take all points will give opportunity for Spurs back to champion league next time. I think 9 match left, Tottenham Hotspur just have face with Liverpool as top team against and another left all dominance with bottom position standing team.