Setup for more than 4 players in Dominion?
The official rules for 5-6 players in Dominion is outlined in the Dominion: Intrigue rulebook, as it requires the extra treasure and Province cards included with that expansion.

To set up for 5 or 6 players, combine the Treasure cards from Dominion and Dominion: Intrigue. Use 15 Provinces in the Supply for a 5-player game and 18 Provinces in the Supply for a 6-player game. All other Victory card piles (Estates, Duchies, and Victory Kingdom cards) remain at 12 cards per pile. Use 40 Curse cards for a 5-player game and 50 Curse cards for a 6-player game.
In a 5 or 6 player game, the game ends at the end of any player’s turn when one of two conditions is met: the Supply pile of Province cards is empty OR any 4 Supply piles are empty.

I would recommend using these rules with proxy cards for the extra treasures, Provinces and Curses if you don't have the Intrigue expansion.