Conor Dwyer of the United States was the top qualifier in 3:43.42.

Park entered the Rio Games ranked sixth in the 400m free this year with 3:44.26, while Australian Mack Horton was first with 3:41.65.

Racing in the sixth of seven heats, Park had the fastest reaction time with 0.64 second. He led the pack at the first 50m mark, but dropped to fifth by the 100m. He remained in the middle of the pack and rose to third place at the 250m turn, but eventually finished fourth among eight swimmers in his heat.

Park was 0.20 second back of the last qualifier, Jordan Pothain of France.

Sun Yang of China, the 2012 gold medalist, won that heat in 3:44.23, good for fourth overall among the eight qualifiers.

Park, who has also won two world championships in the 400m, took a dramatic route to Rio.

He was initially barred from the national team because of his doping history: Park tested positive for testosterone supplements in 2014 and served an 18-month ban from September 2014 to March this year.

The Korean Olympic Committee (KOC) made him ineligible for Rio, based on a rule that prevents athletes from representing the country for three years after the end of their doping suspensions.

Park still entered the national team trials in April and met the Olympic qualifying standards set by FINA, the international swimming governing body, in the 100m, 200m, 400m and 1,500m.

Park then appealed the KOC rule at the Court of Arbitration for Sport, and the world's top sports tribunal upheld the appeal on July 8, the deadline to submit the national swimming team entry.

In the aftermath of his early elimination, Park said he wasn't sure how to feel about the situation.

"I am embarrassed to be giving an interview," he said. "I am not even going to the final at the Olympics, and it hasn't hit me yet. I am sorry to fans back home."

Park blamed his subpar race on butterflies in his stomach.

Between his doping suspension and a protracted legal battle, Park got in only two competitions this year -- the national team trials in April and a minor international meet in Australia in June.

"I tried to do the best I can, but I think I might have been too nervous because I hadn't been in a major event since the Asian Games two years ago," Park said. "I've received this precious opportunity and this is disappointing."

As for his race strategy, Park said he felt bogged by nerves, and refused to make his lack of competitive experience an excuse.

"I should have had a better spurt," Park said. "I should've kept pace with Sun Yang."

In the lead-up to Rio, Park often said his focus would be on enjoying himself, since he considered it a miracle that he was even in Brazil at all.

This was Park's first race in Rio. He will also enter the 100m, 200m and 1,500m freestyle in his fourth and likely final Olympics.

With the heats for the 200m freestyle looming Sunday, Park said he will try to regroup mentally the best he can.

"I'd love not to worry about the mental aspects," he said. "I want to make everyone think that I've made the most of the opportunity." (Yonhap)