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.
No.
In the case Manchester United wins the Europa league this season, they will qualify for the champions league and the 5th placed team of the premier league will qualify for the champions league too.
It's even possible that 7 english teams will take part in the champions league next season. That will happen if Aston Villa wins the champions league this season and Manchester United wins the Europa league.