Tanvir Ansari, a doctor, was arrested on Sunday after being questioned for several days in connection with the seven blasts on packed rush-hour commuter trains.
Ansari is an alleged operative of the Lashkar-e-Taiba, which is fighting Indian control in Kashmir and is the group linked by police to the train attacks, the Press Trust of India reported.
Unnamed police sources said Ansari was suspected of having trained in Pakistan in handling arms, ammunition and making bombs. He was due to appear in court late yesterday.
Indian police made their first three arrests last Thursday.
They said they had definitive evidence that the three had links with terrorist groups. But reports on Monday said police did not think they were directly involved in carrying out the attacks.
Two of the men were arrested in eastern Bihar state and the third man from Mumbai. A court on Friday ordered them to be held for 10 days for further questioning.