Police carried out a total of 22 raids in eastern Belgium, searching homes of suspected ring members. They found no weapons, ammo or explosives, according to the prosecutor quoted by Belgium's De Morgen.
The four will face a judge later in the day to decide whether they should remain in custody.
The group of Islamists in Belgium's eastern city Verviers, some of whom had fought alongside Islamic State in Syria, planned to attack police personnel. During the operation two weeks ago, security forces killed two suspects and detained one man.
Belgium has been on high alert following the break-up of the Verviers terror network. Police have stepped up security at sites identified as possible targets, including embassies of countries involved in the military operation against Islamic State in Syria and Iraq.
