Just had a season as a lvl 4 manager, ended my season with my first treble. I noticed i played a semi and a final against lvl 7 managers with 8* and 9* players. In my opinion with max effort and determination it's definitely possible to beat way stronger teams. At first i was frustrated with the huge differences, but as the season advanced with the right training for your key players in key positions it's not that improbable to give them a good run for their money. Just give it all you got and enjoy, you can't win em all!