The man has been identified as 46-year-old Ri Jong-chol.
An Indonesian woman, a Malaysian man and a woman with a Vietnamese passport were detained earlier.
Malaysian police believe poison was sprayed into Kim Jong-nam’s face as he waited to board a flight from Kuala Lumpur to Macau.
Malaysian Deputy PM Ahmad Zahid Hamidi formally confirmed on February 16 that the dead man, who was travelling under the name Kim Chol, was Kim Jong-nam.
Police say Ri Jong-chol was detained on February 17 in Selangor, near Kuala Lumpur. No further details were given.
One of the detained women, an Indonesian national named as Siti Aisyah, is said to have told Malaysian police she had been paid to perform what she thought was a prank.
A Malaysian man thought to be her boyfriend was detained along with her.
The woman carrying a Vietnamese passport has been identified as Doan Thi Huong.
Kim Jong-nam is believed to have been attacked in the airport departure hall on February 13 by two women, using some form of chemical.
A grainy image taken from security camera footage, which has been broadcast in South Korea and Malaysia, shows a woman wearing a white T-shirt with the letters “LOL” written on the front.
It is not clear whether either of the detained women is the woman in the footage, and police say they are still looking for “a few” other suspects.
Police have now finished Kim Jong-nam’s post-mortem examination, though the results have not yet been made public.
North Korea has said it will reject the result of the autopsy.
It has demanded that Malaysia immediately release the body. Malaysia is refusing to do so until it receives a DNA sample from Kim Jong-nam’s next-of-kin.
South Korea’s intelligence agency has accused North Korea of assassinating Kim Jong-nam, saying Pyongyang had wanted to kill him for years but that he was being protected by China.
Despite widespread speculation that North Korea was behind the killing, there has been no proof. Pyongyang has made no public comments on the issue.
Kim Jong-nam was largely estranged from his family, after being passed over for the North Korean leadership in favor of his youngest half-brother. He spent most of his time overseas in Macau, mainland China and Singapore.
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