Beijing Police said the four persons were detained for "spreading obscene content."
The video shot on a smartphone shows a young Chinese couple having sex in a fitting room. The couple admitted having sent the video to some friend on Wechat, a mobile app.
But the video was hacked and posted on the web, drawing fierce criticism from web users.
The offscreen broadcast indicated it was taken in a Uniqlo store in Sanlitun, an upmarket shopping district, in April.
Also Read
It said Sun was suspected of having posted the video on Weibo.Com, a major microblogging platform in China, while the other three were suspected of spreading obscene information.
Police said they were still investigating the couple, who would face punishment if found to have deliberately spread the footage, state-run Xinhua news agency reported.
China's criminal law stipulates that those disseminating obscene books, films, pictures and video clips could face up to two years' imprisonment, while those who make obscene products for profit could be sentenced to life.
Like YouTube, popular networking sites such as Facebook and Twitter are inaccessible in China.
The ruling Communist Party oversees an extensive censorship system dubbed the Great Firewall. It aggressively blocks sites, content and commentary that is pornographic, violent or deemed politically sensitive.