In a separate resolution, PACE warned that failure to comply would lead to maintaining sanctions against Belarus, or barring the country's parliamentarians from attending the assembly's sessions even informally.
The warning came just two weeks after the UN's Human Rights Commission censured Belarus over the disappearances and other rights abuses.
Belarus had its special guest status in the Council of Europe suspended in 1997, amid claims that its constitution was falling short of democratic standards and handing too much power to President Alyaksandr Lukashenka.
Since then, Belarus has been regularly criticized in Strasbourg for its poor human rights record, including harassment of nongovernmental media, restrictions of religious freedom, and reports of random arrests.
All four disappearances, which occurred in 1999 and 2000, are believed to be politically motivated. Although Belarusian authorities deny any wrongdoing, they have persistently ignored calls to conduct independent investigations into the cases.
Greek Cypriot delegate Christos Pourgourides, who authored a report on Belarus that was debated at the assembly before the 28 April vote, said the people responsible for these disappearances should be searched for among the country's top leadership.
"As a criminal lawyer, I have no doubt that these disappearances were ordered at the highest possible level in the establishment of Belarus. I cannot be certain that the order was given by President [Lukashenka] himself, but I am absolutely certain that the order for their abduction was given by people very, very close to the president," Pourgourides said.
In another resolution adopted this week, the Strasbourg-based assembly severely criticized Belarus for the "systematic harassment and intimidations carried out by state officials...against journalists, editors, and media outlets which are critical of the president" or the government.
Russia, which is linked to Belarus by a union treaty, expressed its disagreement over the resolutions and recommendations adopted by the assembly.
Talking to journalists after the vote, Konstantin Kosachev, the chairman of the State Duma's Foreign Affairs Committee, criticized the documents for being "too emotional."
Russia itself has been criticized in the past in Strasbourg for human rights violations in Chechnya.
Although the situation in the breakaway Northern Caucasus republic was not on the assembly's agenda this week, it was nonetheless debated among members of PACE's Political Affairs Committee.
In comments made to RFE/RL's North Caucasus Service, PACE's rapporteur on Chechnya, Andreas Gross, said he plans to visit the region in early June with other members of the Political Affairs Committee. He said he will prepare a report to be debated at the assembly's next plenary session later that month.
"Since I was appointed rapporteur last July, I [have never been] allowed to visit Chechnya, and [there] is no use to make a report based only on journalists' [accounts]. You have to go on a [fact-finding] mission yourself. But now, after one year, I have the impression that the Russian authorities -- and especially the new Russian delegation [here] -- are much more cooperative, and we agreed on a mission [so that] we could make a report," Gross said.
Whether Russian authorities will allow the Swiss delegate to meet Chechen separatist President Aslan Maskhadov -- as he says he intends to -- remains unclear, however.
The situation in Armenia, where President Robert Kocharian and his coalition cabinet are engaged in a bitter standoff with opponents, was also debated this week in Strasbourg.
Armenia's parliamentary opposition accuses Kocharian of rigging last year's presidential and legislative polls and insists his leadership should to be put to a vote of national confidence.
The Armenian capital, Yerevan, has witnessed daily opposition rallies for nearly three weeks now. Tensions bubbled over on 13 April when police rounded up dozens of opposition activists and raided opposition party offices.
The crackdown was strongly criticized by Council of Europe Secretary-General Walter Schwimmer, who regretted the absence of democratic debate in Armenia.
In a resolution adopted this week, PACE urged the Armenian leadership to refrain from any actions that could be seen as attempts at curtailing freedom of expression and movement. It also called for an investigation into the recent incidents.
While reiterating its "profound disappointment" at last year's "flawed" elections, the assembly also urged Kocharian's opponents to strive to achieve their goals "within the constitutional framework" and called upon both sides to enter into a dialogue "without preconditions."
Armenia was admitted into the Council of Europe in January 2001, along with its neighbor Azerbaijan.
Although neither country met democracy standards, the Strasbourg-based body hoped that opening its ranks simultaneously to the rival nations would help them reach a solution to their territorial dispute over Nagorno-Karabakh.
Despite a 1994 cease-fire, Yerevan and Baku remain technically at war over the predominantly ethnic Armenian enclave. For various reasons, both sides have rejected successive settlement blueprints drafted by the Minsk Group, the 13-member group of nations mandated by the Organization for Security and Cooperation in Europe to mediate in the talks.
Addressing the PACE assembly yesterday, Azerbaijani President Ilham Aliyev reiterated his country's traditional stance, which consists of demanding that ethnic Armenian troops withdraw from all Azerbaijani lands they have been occupying since 1993, prior to any discussion on the status of Nagorno-Karabakh.
Aliyev succeeded his then ailing father last October following a controversial presidential election marred by irregularities, street violence and the subsequent arrest of opposition activists.
During his three-year tenure as his country's chief PACE delegate, Aliyev often had to adopt a defensive position amid criticism of Baku's poor human rights record. Yesterday, however, his first address to the assembly as Azerbaijani president was delivered in a much more cordial atmosphere.
Aliyev hinted that he might release all inmates that the Council of Europe insists are political prisoners. However, when asked whether he thought he could do so before PACE's September session, the Azerbaijani leader remained noncommittal.
"When I was elected, in my first speech after my inauguration, I said I would be the president of all Azerbaijanis -- and that is what I am doing. The policy of putting an end to the dramatic history of the past will continue, but it is very difficult to do that alone. All political forces must take an active part in doing that. The steps that I have taken in pardoning prisoners show that intention and that policy, and I think that that policy will continue," Aliyev said.
Last month, Aliyev signed a decree amnestying nearly 130 prisoners, including Suret Huseynov, a former prime minister who had been sentenced to life imprisonment in 1999 on charges of plotting against the state.
Huseynov's release brought down to five the number of political prisoners that the Council of Europe wants Azerbaijan to release in the coming months.
In the meantime, an estimated 100 opposition activists detained last October have been charged over their alleged participation in postelection violence. Some of them have already been convicted, while others are still awaiting trial.
Aliyev yesterday justified the crackdown on the opposition, describing it as protection against the "hostility" that he says continues to exist in Azerbaijani society.