Chelsea aren't going to finish in the top four, the side has too many problems and relies on youngsters too much to be consistent. There'll be ups and downs through most of the season.
United has good enough squad to finish 3rd/4th. They're not fun to watch at the moment but they're solid and deep enough to last the distance. I have a feeling Arsenal will miss out, unless Spurs fall apart, which I don't rule out, either, because I think this Tottenham side is, at best, a season away from crumbling. Too many false dawns and near wins for them in the recent years, it's a familiar recipe for an impending crisis. But I still think overall they have a better team than Gooners, so i's a coin toss, really.