The fourth position isn't the problem anymore but the 5th position because UEFA has added one more slot to make it five clubs qualifying from EPL to the Champions League next season. It's only if United wins Europa League that only the top four clubs on the table at the end of the season will qualify to UCL. Let's see amongst these three teams which of team wouldn't make it to UCL next season.
Sure, just the same thing applicable to Europa league because chelsea would have been part those teams that is competing in the Europa league this season, if not for the fact that manchester united won the FA cup title. Which is why chelsea couldn't make it up to be in the Europa league this season, they only qualified to be in the conference league. However, I think you're right when you said that manchester united performance will determine if there will still be a hope for the team that will end in the 5th position.