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.
Arsenal is not a team to rely on to such extent, we've seen it lots of times when they blow up their chances of being in top four, when you least expect them to deliver they tend to fail then, but when you are not expecting much from them they will deliver, no one expected them to have a fine run both home and away before they met Man city recently, the top 4 is still wide open in my opinion
For the first four, of course, the claims of different teams continue. However, Arsenal is the most ambitious of these teams. There is a point advantage. It's easier for me to set up. I don't see a team that will force them for fourth place. I think Arsenal will be in the top four teams of this year. That's how it goes if they don't get back-to-back losing streaks.