Redwan El-Ghaidouni, 38, was shot eight times through his car window in Uxbridge, west London, last year.
A 36-year-old man was arrested on suspicion of conspiracy to murder on Friday and released on bail until mid-June pending further inquiries.
It follows the arrest of a 41-year-old man, a 29-year-old man and a 34-year-old woman on Wednesday.
They were also held on suspicion of conspiracy to murder and bailed until June.
Father-of-three Mr El-Ghaidouni was shot after pulling into his driveway at about 19:00 GMT on 3 February 2015.
He was killed at the home he shared with his partner and three children on the junction of St Andrews Road and Vine Lane.
Police believe there were failed attempts to shoot Mr El-Ghaidouni on 31 January and 2 February 2015.
"As we've previously said, Redwan did have a criminal past and had been linked to drug importation," Det Ch Insp McHugh Noel said.
"He had been released from prison almost seven months before his death but he had been working full-time at a car dealership and enjoying being a family man again.
"We continue to look into his background for answers about what happened but whatever the circumstances, his murder has left three young sons without a father and his long-term partner devastated."
A Â£50,000 reward is being offered for information.
A fourth person has been arrested in connection with the death of a man who was shot dead on his driveway.