Big Brother 2004, also known as Big Brother 4, was the fourth season of the Australian reality television series Big Brother Australia, and was aired on Network Ten, starting on Sunday 2 May 2004, with the housemates entering the day before, and ended on Monday 26 July 2004, lasting 86 days. The season was billed as "back to basics, but with one small secret". In a return to the basic format of Big Brother 1 and Big Brother 2 that had been eschewed in favour of twists and surprises in Big Brother 3, all housemates were let into the house together and former surprise elements such as swimming pools and gym equipment were all exposed from the beginning.
Two Intruders entered the house later in the game, and Miriam, the transsexual star of the show There's Something About Miriam, entered as a houseguest near the end. The prize money was raised from $250,000 to $1,000,000, but this was kept hidden from housemates until later in the series. Evicted housemates also received larger prizes than any season before, with every evictee receiving a top of the range Mitsubishi Lancer VR-X. The two Intruders were some of the least liked in all the four series, with Violeta being chosen for eviction by every single housemate, and then Monica, who already had a boyfriend outside, immediately started falling for Ryan, and she and Bree developed rivalry towards each other. When Monica was finally up for eviction, she was evicted with a large percentage of the votes. The final contestants in 2004 were Bree Amer and Trevor Butler. Trevor won, and then proposed to his girlfriend on stage. Mid-season, The housemates were able to see the unofficial premiere of The Day After Tomorrow.