The four who were arrested all work for a local culture promotion company that operates iListen.cn, one of China's largest online audiobook providers. The site is currently offline.
They were seized on suspicion of "spreading pornography and harming the morals of young people" after about 1,000 files available for download on the site were found to contain unsuitable material, according to the Shanghai municipal public security bureau.
The company made its own material by recording young women reading erotic novels and also encouraged netizens to upload pornographic audio documents by promising them a share of the profits, officials said. The website, which charged customers to download the offending audio books, has reportedly made more than $5,800 since it launched the service early last year.
With a registered membership of more than 500,000, the website is "potentially hazardous to the online community with its unhealthy audio documents designed to attract visitors and increase earnings," said Qu Weifang, director of the information security division of the public security bureau.
Qu also said that it was more difficult to find pornographic audio material than it was to find videos using traditional search engines, so, "We will step up efforts to examine audiobook websites to ensure a healthy online environment."
The bureau also will expand the cleanup campaign to include cellphone websites, chatrooms and instant messenger groups.
The nationwide crackdown launched by China's ministries for public security and culture targeting websites that offer erotic material started in January. More than 1,900 sites have been shut down, the national Internet regulator said in a statement last month. About 500 online service providers and 160,000 websites based in Shanghai have been encouraged to take more care with their security systems.
Qu said: "We'll also educate the public to raise awareness of resisting such content and encourage them to monitor online pornography."