The dismissed officers went on trial for "bending the law to show favouritism" by shielding Bo's wife, Gu Kailai, from an inquiry into the death of Briton Neil Heywood.
Gu stood trial for poisoning the businessman over a financial transaction that went sour, according to a court statement. She did not dispute the murder charge during yesterday's seven-hour, closed-door trial hearing and a verdict will soon be delivered, the statement said.
Heywood's death in November and its alleged cover-up in Bo's stronghold of Chongqing, the southwestern municipality he ran, was central to the torrent of events that toppled him from the Politburo and exposed the ruling Communist Party to its worst upheaval in decades.
The party's priority now is ensuring top-down control before a handover of power to a new generation of leaders this year.
The legal noose is tightening fast on Bo's wife and police involved in investigating the murder case, suggesting there is a danger Bo could himself face charges of masterminding a cover-up and could risk a lengthy jail term.
The South China Morning Post said on Friday that Bo's former Chongqing police chief, Wang Lijun, would stand trial as early as next week in the southwestern city of Chengdu. Wang sought temporary refuge in Chengdu's US consulate in February after sources said he told Bo that Gu was a murder suspect.
Wang's dramatic flight to the US mission triggered the murder scandal that quickly led to Bo's downfall. Until then, Heywood's death had been officially attributed to a possible heart attack brought on by excessive alcohol consumption.
Chinese media stuck to the terse official account of Gu's trial today, despite avid public interest in this scandal that has exposed the fusion of wealth and privilege in China's political elite, and exposed rifts in the party.
Bo, 63, has not been a focus of the proceedings so far. But most experts believe the trial and almost certain conviction of his wife Gu and the four police officers is a prelude to his punishment, which could include a criminal trial.
VERDICT WON'T BE DELAYED
The court in the eastern Chinese city Hefei did not say when it would announce any verdict against Gu. But the usual wait was about a fortnight said Chen Guangwu, a criminal defence attorney who has followed the Chongqing scandal closely.
"But they won't delay for too long, because this case is being heard in order to pave the way for dealing with Bo Xilai himself," said Chen, who is based in eastern Shandong province.
"This case is in part about testing the waters for that. That is, they will sentence her and see what reaction there is in society and public opinion."
Bo's downfall has stirred more public division than that of any other party leader for over 30 years.
To leftist supporters, Bo became a charismatic rallying figure for efforts to reimpose party control over dizzying and unequal market growth. To liberal critics, Bo was a dangerous opportunist who yearned to impose his harsh policies on the entire country.
As the four sacked officers went on trial, also in Hefei, Chinese police cordoned off the courthouse and excluded foreign reporters from the hearing. Vans parked nearby were bristling with video surveillance equipment.
A court spokeswoman said the case would start at 8.30am (local time).
"It's open to the public but I'm afraid all the places are full at this time," she said.
The four men on trial - Guo Weiguo, Li Yang, Wang Pengfei and Wang Zhi - were senior police officers in Chongqing who allegedly sought to stymie an investigation into Heywood's death in a hilltop hotel villa overlooking Chongqing.
Yesterday, a court official said prosecutors believed Bo's wife, Gu, and a family aide, Zhang Xiaojun, killed Heywood by pouring poison down his throat after a business dispute led Gu to believe Heywood had threatened her son, Bo Guagua, then a student at Harvard University.
During Gu's seven-hour hearing yesterday, it was alleged Heywood had written a letter to Guagua, threatening to "destroy" him, said a source who had been briefed on the hearing.
Heywood and Guagua had fallen into dispute over Heywood's demand for a fee to help arrange a £130 million ($NZ250 million) financial transaction, the source added.
Guagua, believed to be in the United States after graduating this year from Harvard University, denied there was such a deal of that value but appeared to confirm the letter's existence.
"I cannot comment on any of the details (of the letter), but I can disclose that there is no such thing as either possessing or transferring £130 million," Guagua said in an e-mail sent to Reuters.
