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.
The 4th position this season is in the hands of Arsenal and I'm not sure Manchester United and Tottenham will manage to overtake Arsenal until the end of the season. Arsenal's good form can be expected to stay safe in the 4th position at the moment, they have a few points advantage over Manchester United and Tottenham and Arsenal also have 1 game in hand. So it will be good to keep thinking Arsenal will be safe in 4th place this season.
West Ham, Tottenham and Manchester United will compete for the 5th place as Europa League contestants. Maybe Manchester United will take this ticket later even though their performance is not consistent.