Jeff Olsen, an unheralded 16-year-old, swept the National Sports Festival up in his lengthy arms tonight by winning his fourth swimming gold medal in three days at the Louisiana State University Natatorium.

Olsen, of Austin, Tex., beat local favorite Andre duPlessis in a touch-and-go thriller to win the men's 400-meter freestyle in a time of 4:01.19, ninth fastest in Festival history.

In the women's division, Molly Magill, a 14-year-old from Largo, Fla., won her second gold medal by easily outdistancing a youthful field in the 1,500-meter freestyle with a time of 16:51.84, fourth fastest in Festival history. She won the 800-meter freestyle Saturday and got a bronze in the 200 freestyle Sunday.

In the men's 400 freestyle, part of the crowd was for duPlessis, a 16-year-old from New Orleans, and part rooted for Olsen, the 6-foot-4 teen-ager who has charmed the Festival with his talent and a wingspan reminiscent of West German Olympian Michael Gross.

Olsen, who swims with a long, almost lazy stroke, maintained a smooth pace through the first 200 meters of the race, hovering in third place behind Tim Gaulladet of Westlake Village, Calif., and duPlessis. He took the lead at the 250-meter mark. DuPlessis chased him down the stretch but Olsen's reach made the difference. He touched the wall four-tenths of a second ahead, duPlessis finishing in 4:01.59.

Gallaudet finished in 4:05.92 for the bronze.

"After I caught Gallaudet, the race went pretty much the way I thought it would," Olsen said. "I felt duPlessis and actually saw him out of the corner of my eye. I thought of how hard I trained for this and really started pulling. The last 50 meters he pushed me harder than anyone has yet."

Olsen won the men's 200 freestyle on Saturday and the 800 freestyle and 200 butterfly Sunday. He had a chance at a record-tying fifth gold medal in the men's 800-meter freestyle relay tonight, but his West A relay team finished sixth.

Olsen was at the Festival to prepare for the national championships Aug. 5-9 at Mission Viejo, Calif. He will swim the 200 butterfly and the 400 and 1500 freestyle events there.

Olsen was the subject of some irony Sunday night after he won his third gold medal. The U.S. Olympic Committee is choosing athletes at random for drug testing, an unpopular decision among many of the competitors. Olsen was selected and given an hour to report. But Olsen was scheduled to swim in the 200 butterfly later in the evening, so he was given an extension. It turned out the USOC also was planning to test the winner of the 200 butterfly, and Olsen saved another athlete a test by winning the race.

But when he got to the test center, the USOC then realized he is only 16. The USOC is not allowed to test anyone under 18, thus didn't get a test at all, to the delight of many of the athletes.

"I didn't know there was any age limit," said the high school junior. "They told me to go watch the cycling."

Today's swimming events coincided with the one-year anniversary of the opening of swimming events at the 1984 Summer Olympics in Los Angeles, a day on which Carrie Steinseifer and Nancy Hogshead tied for the gold medal in the 100-meter freestyle, the first tie in Olympic swimming history. In recognition of the anniversary, Hogshead made a guest appearance and presented the medals at Olsen's ceremony.

Magill is a high school junior who doesn't hold anything back -- she frankly states that she will make the 1988 Olympic team. She never was threatened in the distance race by silver medalist Jennifer Linder, 16, of Des Moines, Iowa, who finished in 17:04.72, or bronze medalist Katherine Creighton, 15, of Cincinnati, who came in at 17:13.87.

"I could tell I was pretty far ahead," Magill said. "It hurt a lot, I was going for a good time. I didn't really come here looking for gold medals. I just wanted to see if I could place in the top three."

Magill's fast time pushed the other swimmers. Linder's time was the eighth fastest in Festival history, Creighton's the 10th.

Another superb time was turned in by 13-year-old Jeanne Gibbons of Wayne, Pa., who won the women's 100-meter freestyle in 58.11, fourth fastest in Festival history. Colette Vanderburg, 16, of Barrington, Ill., won the silver in 58.62, and 14-year-old Cathy Ritch of Mt. Kisco, N.Y., winner of the 100-meter backstroke here, finished third in 58.70.

In other swimming events, 15-year-old Karen McClure of St. Louis won the women's 200-meter breaststroke in 2:39.18, third fastest time in Festival history. Mary Ruppe, a 16-year-old from Mishawaka, Iowa, won the silver in 2:41.55, and Jenny Toten, a 14-year-old of Novato, Calif., took the bronze in 2:42.96.

Mike Dobbs, of Howard Beach. N.Y. won the men's 200-meter breaststroke in 2:06.19. David Meck, a 17-year-old from Campbell, Calif. who won the 100-meter backstroke Sunday, took second in 2:06.62, and Jeff Rouse, a 15-year-old from Fredericksburg, Va., won the bronze in 2:09.72.

The South men's basketball team, led by guard Mike Porter's 18 points, scored an 85-81 victory over the previously undefeated North team. Porter, a 6-footer from Pulaski, Va., had eight of the South's final 13 points.

In women's basketball, the South beat the North, 78-73, as Alisha Jones of Louisiana State had 19 points and seven rebounds. The South (2-1) advanced to the final against the East (3-0), which beat the West, 56-53.

In the East's victory, Dawn Royster scored 10 points as her team took control late in the game. The West (1-2) led for all of the first half and by three points with four minutes to play.

A basket by the University of Maryland's Carolin Dehn-Duhr with 2:42 to go gave the East the lead at 54-53 and Royster, from North Carolina, scored an insurance basket in the final minute.

Karon Howell led the West with 10 points. Penny Toler, formerly of St. Anthony's High School in Washington, D.C., and Cherie Nelson scored eight points each.