The request comes from a lawyer for Davontae Sanford, an 18-year-old who is trying to get his conviction overturned in those four deaths.
Vincent Smothers is in prison for killing eight people. Police say he also confessed to four other killings but hasn't been charged.
Sanford believes Smothers would clear him if he testified in court. Sanford's attorney, Kim McGinnis, wants a judge to order prosecutors to offer immunity to Smothers so his words could never be used against him.
The Wayne County prosecutor's office opposes the request. A court hearing is set for Friday.