The six men and six women will gain access to the bar if they complete set tasks successfully. Previous tasks have included completing obstacle courses and games of "keepy uppy".
It has also been reported that the 12 contestants will be served by minor guest celebrities. The show is again being presented by Davina McCall, who has hosted all three previous series to date.
Channel 4 already knows that giving alcohol to the contestants leads to compulsive viewing as the housemates get a mixture of amorous, flirty and rowdy. The last series saw tabloid favourite Jade Goody strip naked.
Channel 4 is obviously hoping the that boozy atmosphere will lead some housemates to go further. The Sun newspaper has already offered £50,000 for the first heterosexual couple to have sex on the show.
It is not clear whether the contestants will immediately be given access to the bar or whether access will only be granted later during the nine-week run.
The bar has been made to look like a traditional "East End boozer", kitted out with beer pumps and 1970s-style pictures of dogs, and it is hidden behind a wall at the back of the bathroom.
However, while there will be plenty of beer and wine, the 'Big Brother' bar will not carry any spirits.
The 'Big Brother' house in Elstree, Hertfordshire, has been completely rebuilt for the fourth series at an estimated cost of £10m. It will not include, as did the last season, a rich and a poor side, which led to much tension between the housemates.
Phil Edgar Jones, executive producer of 'Big Brother', said: "This year we wanted the housemates to live in a more natural and friendly place."
Big Brother begins tonight at 8.30pm, with an update at 10.45pm.
Last year's show was won by Kate Lawler, who now hosts a radio show on Capital and co-presents Channel 4's wilting breakfast show 'RI:SE'.
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