They have also released new footage showing the suspects in the area shortly before the killing.
Police believe James was shot accidently in a dispute between local gangs.
Det Ch Insp Katherine Goodwin said: “Four years have passed since James’ life was cruelly taken from him. His family have had to live with the pain and torment of knowing his killer is still out there for all these years.
“They have not given up, and neither have we, and we are more determined than ever that we will catch those responsible for this terrible crime.”
She added: “It has been four years but we are still actively investigating. For the community it may feel like a long time but it doesn’t feel a long time for James’ family or friends.