HouseGuests are sequestered in the Big Brother House with no contact to or from the outside world. Each week, the HouseGuests take part in several compulsory challenges that determine who will win food, luxuries, and power in the House. The winner of the weekly Head of Household competition is immune from nominations and must nominate two fellow HouseGuests for eviction. After a HouseGuest becomes Head of Household, he or she is ineligible to take part in the next Head of Household competition. HouseGuests also take part in food competitions in which they must win in order to eat regular foods. Losers of the competition are put on a peanut butter and jelly diet. The winner of the Power of Veto competition wins the right to save one of the nominated HouseGuests from eviction. If the Veto winner exercises the power, the Head of Household must then nominate another HouseGuest for eviction.
On eviction night, all HouseGuests except for the Head of Household and the two nominees vote to evict one of the two nominees. This compulsory vote is conducted in the privacy of the Diary Room by the host Julie Chen. In the event of a tie, the Head of Household must cast the deciding vote, announcing it in front of the other HouseGuests. Unlike other versions of Big Brother, the HouseGuests may discuss the nomination and eviction process openly and freely. The nominee with the greater number of votes will be evicted from the House on the live Thursday broadcast, exiting to an adjacent studio to be interviewed by Chen. HouseGuests may voluntarily leave the House at any time and those who break the rules may be expelled from the house by Big Brother. The final seven HouseGuests evicted during the season will vote for the winner on the season finale. These "Jury Members" will be sequestered in a separate house and will not be allowed to watch the show except for competitions and ceremonies that include all of the remaining HouseGuests. The jury members will not be shown any Diary Room interviews or any footage that may include strategy or details regarding nominations.
Some changes from previous seasons were implemented throughout the course of the game. The main twist for the year was The X-Factor. Eight HouseGuests were introduced, only to have five HouseGuests' exes compete against them in the game. The twist ultimately led to the expulsion of HouseGuest Scott, who had a violent outburst and was removed from the game. Another change in the format, introduced in the last veto contest of Big Brother 3, was the Golden Power of Veto, in that if a nominated person won such a veto, they would be allowed to take themselves off consideration for voting, which was not allowed when the veto was introduced originally the year before. Since its inclusion in Big Brother 4, The Golden Power of Veto has appeared in all seasons afterwards, with only a few changes made. One last twist to the format included a jury of seven who would decide the outcome of the game. When the game got down to nine players, the evictees formed the jury, which eliminated the need for America to break a tie if necessary. Whereas, prior to this season, evictees were sent back home and could watch the series freely, jurors were placed in a sequestered house, and saw only competitions, ceremonies, and evictions.
Thirteen HouseGuest were cast for this season of Big Brother. One of the original selections, Brandon, was kicked off of the show during the sequester period, after it was revealed he had communicated with his girlfriend. After his removal from the game, Jee was sent in as his replacement. As part of this season's twist, five of the original eight Houseguests were shocked to find that an ex would be playing the game with them, as part of the "Ex-Factor" twist. The exes were Jun and Jee, Alison and Justin, Erika and Robert, David and Michelle, and Amanda and Scott.
Alison Irwin competed in The Amazing Race 5 with then-boyfriend Donny Patrick, and finished in 10th place. Irwin and fellow Houseguest Erika Landin returned to compete in Big Brother 7: All-Stars, where Irwin placed 14th and Landin was the runner-up. Dana Varela was also a candidate for Big Brother: All Star but was not selected to compete. Jun Song and Jack Owens both made a brief appearance on Big Brother: All Stars during competitions. Song also made an appearance on Big Brother 10 to host a food competition along with other Big Brother alumnus.
On Day 1, Alison, Dana, David, Erika, Jack, Jun, Nathan, and Scott entered the Big Brother House. Upon getting to know one another, they competed in the "X Marks the Spot" food competition, in which everyone successfully earned food for the first week. Upon returning inside, they were informed of the X-Factor twist, in which five more HouseGuest would be entering the game, and would be exes of some of the eight already in the house. Following this announcement, Amanda, Jee, Justin, Michelle, and Robert entered the House. On Day 3, HouseGuest partnered up and competed in the "Who's Ya Chum?" Head of Household competition. Though Alison and Nathan won the endurance portion of the competition, Scott chose which of the two would become the first HoH; he chose Nathan. On Day 5, he nominated Amanda and Jee for eviction. On Day 7, Dana won the "Feeling Knotty" Power of Veto competition, and chose to keep nominations the same. On Day 8, Scott was expelled from the game after a violent outburst, in which he revealed he had a sexually transmitted disease. On Day 12, Amanda was evicted in a unanimous vote.
Following Amanda's eviction, the remaining HouseGuest competed in the "Majority Rules" HoH competition, in which they had to answer questions on how they felt the majority of the HouseGuest would vote. Jee was the winner of the competition. The following day, Michelle, David, Robert, Dana, and Nathan were put on the peanut butter and jelly diet, after losing the "Clash of the Casseroles" food competition. Jee nominated Erika and Michelle for eviction on Day 13. On Day 14, David won the "Duck Ball" competition, and chose to leave nominations the same. On Day 19, Michelle was evicted in a six to two vote, only receiving the votes of Justin and Robert.
Following Michelle's eviction, HouseGuest competed in the "Everything In Three's" Head of Household competition, in which they faced off three at a time answering questions. Dana was the winner of the competition. On Day 20, all of the HouseGuest earned food for the week after the "Paratrooper" competition, in which they had to catch falling commando dolls in military hats. Each doll had the name of a food on it, and if they got the doll they would earn that food for the week. Later that day, Dana nominated Alison and Jack for eviction. The following day, Nathan won the Power of Veto, and used it on Alison. David was named her replacement nominee. On Day 22, Alison won the "Spin-O-Matic" luxury competition, earning her and another HouseGuest a luxury dinner. She chose to take Nathan with her. On Day 26, David was evicted in a five to two vote, only receiving the votes of Erika and Alison.
Following David's eviction, Alison won the "Around the Water Cooler", in which HouseGuest had one minute to stack as many cans as possible by dropping them into a tube. Dana, Erika, Jee, and Robert were put on the peanut butter and jelly diet, after losing the "Laying Pipe" food competition. Alison chose to nominate Dana and Jun for eviction. On Day 28, Robert won the "Snake In the Grass" Veto competition, and chose to leave nominations the same. On Day 33, Dana was evicted in a unanimous six to zero vote, and become the first HouseGuest to enter sequester to form the Jury of Seven, in which the seven HouseGuest would vote for the winner of Big Brother 4 on the finale night.
Following Dana's eviction, HouseGuest competed in the "Who Said It?" Head of Household competition, in which HouseGuest tried to guess which evicted HouseGuest made a certain statement while in the house. Justin won the competition. The following day, everyone earned food for the week during the "50 Ways to Cook a HouseGuest" food competition. Justin nominated Jack and Nathan for eviction. On Day 36, Robert won the "Quoridor" Power of Veto competition, and chose not to use the Power of Veto on either Jack or Nathan. On Day 40, Nathan was evicted in a unanimous vote, becoming the second member of the Jury of Seven.
Following Nathan's eviction, HouseGuest competed in the "Steel Cage Match" endurance Head of Household competition, in which HouseGuest had to stay inside a steel cage, and were eliminated if they left. The following day, everyone earned food after the "Clambake From Hell" competition. Erika nominated Justin and Robert for eviction later that day. On Day 43, Jun won the "Video Veto" Power of Veto competition, and chose to leave nominations the same. On Day 47, Justin was evicted in a three to one vote, only receiving the vote of Jee. He was the third member of the Jury of Seven.
Following Justin's eviction, HouseGuest competed in the "Black HoHle" Head of Household competition. Jee was the winner of the competition. He nominated Erika and Jack for eviction. The following day, Jee won the Power of Veto, after forcing the other HouseGuest to be put on the peanut butter and jelly diet for the week. He chose not to use the Power of Veto, leaving his nominations intact. Despite Jee wanting Erika evicted that week, Jack was evicted in a two to one vote, only receiving the vote of Robert. This made Jack the fourth member of the Jury of Seven.
Following Jack's eviction, HouseGuest competed in the "Disappearing Act" Head of Household competition, which Jun won. Jun, as HoH, won a special trip outside of the Big Brother House to attend the 20th Annual MTV Video Music Awards. Her disappearance (as she entered the Diary Room and never returned) played a part in the veto Competition, as the HouseGuests had to guess where Jun was. Jun returned, and nominated Jee, her ex-boyfriend, and Alison. However, Alison won the veto, and used it on herself, forcing Jun to put Robert in her place. Despite Robert being a replacement for Alison, Jee was unanimously evicted, making him the fifth member of the Jury of Seven.
Following Jee's eviction, Robert won the "Dearly Departed" Head of Household competition. The following day, everyone earned food during the "Mexotic Dinner" food competition. Robert chose to nominate Alison and Jun for eviction. The following day, Alison won the Diamond Power of Veto, which was the final Veto competition of the season. With the Diamond PoV, Alison removed herself from the block, and Erika was named her replacement. Alison then cast the sole vote against Erika on Day 68, making her the sixth member of the Jury of Seven. The final Head of Household competition was in three parts. Alison won the first Round, the endurance "On the House" competition. In the second part, Jun beat Robert in the "Unexpected Relations" competition, a challenge based on events that had taken place in the house. Alison and Jun then faced off in the Final Round of the HoH. After eight questions based on their ex-boyfriends Jee and Justin, the women were tied on four correct answers apiece. The tiebreaker question was "How many days did you date your ex for?", to which Alison answered zero days and Jun answered one million. Therefore, Alison emerged as the final Head of Household for the season and chose to evict Robert from the house. On Day 82, Jun was crowned the winner of Big Brother 4 in a six to one vote, with Alison only receiving the vote of Nathan.
Ten HouseGuests entered the house with an ex-parther, with whom he or she broke up prior to the show. These exes are indicated by matching colors in the chart. Beyond the personal effects of their histories and relationships, there was no direct effect on the mechanics of the game; exes were free to work with or against each other as they saw fit.^Note 1 : Scott was expelled on Day 8 after he had a violent outburst in the house, however nominations and evictions continued as usual.
^Note 2 : The Golden Power of Veto, introduced in Season 3, replaced the regular veto beginning with this season.
^Note 3 : In Week 8 the Veto competition was held before Jun made her nominations, Jun won a trip outside the house and as part of the competition the other HouseGuests had to guess where Jun was.
^Note 4 : The Diamond Power of Veto was introduced, Alison won the Diamond Power of Veto and the power to remove herself from the block, and choose the replacement nominee.
^Note 5 : Votes in the finale are cast for the HouseGuests choice to win, not for who to evict. This season introduced the jury of seven; the last seven Houseguests evicted from the game voted to decide the winner. This eliminated the need for America to break a tie.