Big Brother 2004, also known as Big Brother 4, was the fourth season of the Australian reality television series Big Brother, and was aired on Network Ten, starting on Sunday 2 May 2004,^[1] 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.

The only main twist of the series was the prize money - which was raised from $250,000 to $1,000,000, but this was kept hidden from housemates until later in the season. Evicted housemates also received larger prizes than any season before, with every evictee receiving a top of the range Mitsubishi Lancer VR-X.

Two Intruders entered the house later in the game, and Miriam Rivera, the transgender star of the show There's Something About Miriam, entered as a houseguest near the end.^[2] The two Intruders were some of the least liked in all the four seasons, 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. Mid-season, the housemates were able to see the unofficial premiere of The Day After Tomorrow.

The final contestants in 2004 were Bree Amer and Trevor Butler. Trevor won, and then proposed to his girlfriend on stage. The winner announced of the finale was watched by 2.864 million Australian viewers.^[3]

The opening sequence was similar to the 2003 opening sequence with some minor sound changes. The background colour is more gradient than the previous season. A scene of a green backyard appears between the diary room and kitchen scenes. The diary room chair is orange instead of red, and the evicted housemate in the titles was changed to blue. Lastly the 84 days was changed to the series 86 days. The theme also was slightly remixed with the introduction of a guitar component.

The first Housemate in each box was Nominated for Two Points, and the second Housemate was Nominated for One Point.

Housemate Merlin Luck staged a protest at his live eviction show. Upon entering the arena, he revealed a makeshift banner which read "Free Th Refugees". The letter E had fallen off and it was intended to read "Free The Refugees" in reference to Australian Government policy of mandatory detention of asylum seekers. Merlin placed gaffer tape over his mouth and refused to speak during the live show. The following night, when he returned to appear as a guest on the Nomination show, he apologised to Killeen for putting her on the spot and making her job difficult by refusing to speak during the eviction show. On this show Merlin spoke about his actions in the highest rating show of the timeslot for that night.^[2]

The producers were criticised for making a deal with housemate Ryan Fitzgerald who was permitted to wear a T-shirt that was branded with "MLS" (acronym for Massive Loser Squad). The T-shirts were being sold while the show was running and Ryan's friends admitted that a proportion of the profits was going towards votes to keep him in the house, thus giving him what was perceived to be an unfair advantage. However the producer pointed-out that many housemates had companies or local communities rally support for them, which was little different, and that there was nothing stopping any other housemate's supporters doing a similar thing to Ryan's friends. Ryan has instead insisted on his radio program that his friends were not using the money to buy votes and instead spending it on alcohol.^[4]

Housemate Bree Amer was incorrectly evicted instead of Wesley due to a vote counting error. The error was discovered after the Sunday night eviction show. Bree had already met up with friends and family. She was returned to the house the following night (Monday).^[5] Wesley was evicted in a special show on the following Thursday.