The person "is suspected of being involved in the disappearance of the tourist," 18-year-old Natalee Holloway, said a statement from the Aruban attorney general's office.
The latest person in custody was born on July 21, 1978, and has the initials S.G.C., the statement said, adding, "As a result of the ongoing investigation, this is all the information that can be given at this time." CBS News Correspondent Peter King reports the suspect is a man.
"A fourth arrest has been made, and his name is 'Steve,' and that's all the information that I have," Natalee's mother, Beth Holloway Twitty, told the Associated Press. "Whether or not he's involved with my daughter's disappearance, I can't wait to find out that answer."
Meanwhile, prosecutors have gone back to court to ask that three other suspects stay in jail, although they haven't been charged: 17-year-old Joran van der Sloot, the son of a justice official on Aruba, and his two friends, Surinamese brothers Deepak Kalpoe, 21, and Satish Kalpoe, 18. The three were detained on Thursday. Under the law, a judge must review their case after 10 days and decide their status.
"The process allows for a period of time for the prosecutor to do the investigation. That can go well over 100 days," said local attorney Lincoln Gomez.
A judge was expected to rule as well Friday on another petition, from van der Sloot's father, Paul van der Sloot, to be able to visit his son in jail. None of the four detainees has been formally charged with a crime.
Authorities' announcement of the arrest Friday came nearly three weeks after Holloway's disappearance in the early morning hours of May 30, the same day she was to return from a five-day trip with 124 other students celebrating their high graduation in Mountain Brook, Alabama.