Three men aged 34, 41 and 45 and one woman aged 34 were arrested last night.
They are currently being questioned by detectives at a Belfast station.
The 52-year-old long-serving prison officer, who is married with children, required surgery after an explosive device detonated under the van he was driving in Belfast on Friday morning.
The explosion occurred in the Hillborough Drive area off the Woodstock Road in the east of the city.
