By Steve Gorman

GLENDALE, Calif. (Reuters) - Celebrity socialite Nicole Richie pleaded guilty on Friday to driving under the influence of drugs and was sentenced to four days in jail, a month after TV co-star Paris Hilton completed a three-week stint behind bars.

Richie, 25, the waif-like daughter of singer Lionel Richie who co-stars with Hilton on the reality series "The Simple Life," was ordered to serve her time by Sept. 28, but it was not immediately clear when or where she would be incarcerated.

She also was fined $2,048, placed on three years probation and was ordered to enroll in a court-supervised rehabilitation program for two-time offenders.

Because of a 2003 DUI conviction, Richie had faced a mandatory sentence of up to a year in jail if found guilty in a trial that was set to begin next month.

Appearing calm and composed and wearing a lose-fitting black shift dress with oversized sunglasses, she was accompanied to court by bodyguards and her boyfriend, singer Joel Madden of the rock band Good Charlotte.

Media interest in Richie's case intensified in recent weeks with reports circulating in celebrity magazines and the tabloid press that she is several months pregnant by Madden and plans to marry him. Those reports have not been confirmed.

During Friday's proceedings, Superior Court Commissioner Steven Lubell twice warned Richie that she could end up doing 12 months behind bars if she violates her probation.

"The court views this plea agreement as a contract," he told her. "If you violate the terms of this agreement, you can and you will go to county jail for more time than the court is giving you today."

He added: "Being under the influence of alcohol or drugs while driving is extremely dangerous."

The only words uttered by Richie during the hearing was "guilty" when asked how she pleaded to a misdemeanor count of driving under the influence of drugs, and "yes" when asked if she acknowledged her previous conviction.

All references to alcohol in the latest case were dropped under the plea arrangement.

Emerging from the courthouse in the suburban Los Angeles city of Glendale, Richie walked past a phalanx of reporters and camera crews without saying a word and was whisked away in a sport utility vehicle.

Richie's plea deal stemmed from a traffic stop in December in which police said she was seen driving the wrong way on a Los Angeles freeway. Officers said she failed a field sobriety test and admitted she had smoked marijuana and taken the prescription painkiller Vicodin.

The Dec. 11 stop came six weeks after it was revealed the rail-thin actress had checked herself into a treatment facility to address weight-loss issues. An arrest report at the time said the 5-foot-1 celeb weighed just 85 pounds.

Hilton was released from jail on June 26 after serving 22 days for violating her probation in a drunken-driving case.