Judicial sources on Monday said three people were arrested in Madrid's multi-racial district of Lavapies at the weekend while a fourth was arrested in Getafe, a suburb of the capital. The four suspects were Arabs, state radio reported.
Five other suspects were due in court on Monday over the bombings of four commuter trains that killed 202 people, believed to be the first Islamist strike in the West since the September 11 attacks.
High Court judge Juan del Olmo will question three Moroccans, a Spaniard and a fifth suspect whose nationality has not been disclosed.
He will decide whether they have a case to answer and should remain in custody or be released, the sources said.
Del Olmo last week accused three other Moroccan suspects of 190 murders, 1400 attempted murders and of belonging to a "terrorist" group after a seven-hour hearing. Two Indian suspects were accused of cooperating with a "terrorist" group.
They denied links to al-Qaida and said they were asleep at the time of the bombings.
Del Olmo is not expected to give his ruling on the other five suspects until late on Monday or Tuesday, court sources said. Formal charges against the suspects come later.
The bombings which injured more than 1800 have sparked a security shake-up across the European Union and put Western capitals on alert.