The last four winners of the Champions League:
Real Madrid (17)
Real Madrid (16)
Barcelona (15)
Real Madrid (14)
The last four runners-up:
Juventus (17)
Atleti (16)
Barcelona (15)
Juventus (14)
So that's eight spots occupied by just 4 teams. Other sides reaching the semi's in those four years have been Monaco (17), City & Bayern (16), and Chelsea (14).
The last four winners of the Europa League:
United (17)
Sevilla (16, 15, 14)
That's four occupied by 2 teams. Finalists have been Ajax (17), us (16), Dnipro (15), and Benfica (14).
Could get confusing...