From Bulbapedia, the community-driven Pokémon encyclopedia.
The Sinnoh League (Japanese: シンオウリーグ Shin'ō League) is the regional Pokémon League of the Sinnoh region. The Sinnoh League is composed of eight Sinnoh Gym Leaders and the Sinnoh Elite Four members, along with the Champion. Pokémon Trainers who earn eight Gym Badges by defeating the Gym Leaders of the Sinnoh League qualify to challenge the Elite Four located in the Pokémon League building. Unlike the three Leagues before it, no one in the Sinnoh League uses two of the same species of Pokémon in their team.
In the anime, Trainers who collect eight Badges can enter the Sinnoh League championship competition, known as the Lily of the Valley Conference.
The traditional Pokémon League Championship follows. Trainers must first collect the eight official Gym Badges to be qualified to battle the Elite Four. Gym Leaders are located in different cities and towns, each with their own different type.
The Elite Four in the anime
Once finished, Trainers must face the Elite Four, as usual, then defeat the Champion at the end of the League. Those who challenge the Elite Four must have achieved all eight Badges from that region and face all four and the current Champion consecutively without losing to any of them.
In Platinum, the Elite Four's Pokémon will drastically level up when the National Pokédex has been obtained and Charon has been arrested at Stark Mountain.
The Elite Four reside in the Pokémon League building.