Greek MEP and European Parliament Vice President Eva Kaili was among those previously arrested in the case.
Prosecutors suspect the Gulf state tried to influence parliament decisions with donations of money or gifts.
Local media have reported that the accused country is Qatar, which the Qatari government has denied.
Watchdogs and opposition MEPs have said it could be one of the biggest corruption scandals the European Parliament has ever seen.
Ms Kaili, one of 14 elected vice presidents, has been suspended from the parliament’s Socialists and Democrats Group and expelled from the Greek centre-left Pasok party.
Cash worth about €600,000 ($632,000; £515,000) was seized by Belgian police in 16 searches in Brussels on Friday. Computers and mobile phones were also taken in order to examine their contents.
A total of six people were detained for questioning, two of whom have been released.
“Four individuals have been arrested by the Brussels investigating judge who is leading the investigation,” the Belgian federal prosecutor’s office said in a statement.
“They are charged with participation in a criminal organisation, money laundering and corruption. Two persons have been released by the investigating judge.”
The prosecutor said investigators had suspected that a Gulf state had been influencing economic and political decisions of the parliament for several months, especially by targeting aides.
Local media has named the Gulf state under suspicion as Qatar.
A Qatari government spokesperson told AFP: “We are not aware of any details of an investigation. Any claims of misconduct by the State of Qatar are gravely misinformed.”
The country “operates in full compliance with international laws and regulations”, he added.
A spokesperson for the European Parliament told Reuters that it would not comment on an ongoing investigation, but would work together with the local authorities if needed.
Ms Kaili’s responsibilities as a vice president include the Middle East.
She has been a defender of Qatar in the past.
In a speech last month about human rights during the Fifa World Cup in Qatar, she called the country a “frontrunner in labour rights”, for abolishing kafala, a legal framework used in several Gulf states which human rights organisations compare to modern slavery.
“The World Cup in Qatar is proof, actually, of how sports diplomacy can achieve a historical transformation of a country with reforms that inspired the Arab world,” she said.
She accused some MEPs of bullying and discriminating against Qatar, adding: “They accuse everyone that talks to them or engages of corruption.”
Qatar has previously been accused of corruption, including in its bid to host the 2022 football World Cup. The country denied the allegations and was cleared of corruption by Fifa.__BBC.com