Chikita Madison, whose daughter Renisha Landers, 23, was a victim, said she fought back tears during a brief court hearing in Sterling Heights, a Detroit suburb.
"Our daughters are in heaven," Madison said outside of court. "We'll see them when it's our time."
The bodies of Landers and Demesha Hunt, 24, were found Dec. 19. Six days later, on Christmas, police found the bodies of two other women in their 20s, Natasha Curtis and Vernithea McCrary, in the trunk of a burning car.
Brown said little in court and let his attorney enter a not-guilty plea on his behalf.
Minutes earlier, attorney Jeff Cojocar told reporters that his client maintains his innocence "100 percent."
At least three of the four victims promoted themselves as escorts-for-hire on Backpage.com, which carries classified and personal ads. Investigators believe that's how Brown made contact with them. Phone records show the victims' last calls were transmitted through wireless towers near Brown's home in Sterling Heights, police Det. Mary Whiting told a judge.
Brown has been in custody since May on charges of mutilation of a dead body and arson in connection to the bodies found in cars. Prosecutors didn't charge him with murder at the time but said he was the chief suspect.
Cojocar suggested Brown may have incriminated himself during a 3½-hour recorded interview with Detroit police, and said he would try to get the statements suppressed.
"There are some things that are not favorable that we'll need to attack," Cojocar said.
Detroit police led the investigation for months until determining the women were killed elsewhere. Then-Chief Ralph Godbee Jr. said he was angered that the city's dark, desolate neighborhoods were becoming a place to drop bodies.
Detroit police turned evidence over to Sterling Heights, including an interview with Brown.
The causes of death still are listed as unknown by the Wayne County medical examiner, said Sterling Heights Lt. Kevin Reese.
Before the court hearing, prosecutors met privately with victims' relatives in a small room.
"The parents and relatives of these women have suffered so greatly. We will work to bring closure to these grieving families," Macomb County Prosecutor Eric Smith said in a statement.
Hunt's mother, Denise Reid, said she didn't know anything about her daughter's connection to Backpage.com.
"I'm hearing it like you are," she said. "It's not important to me. It's not relevant. It still doesn't justify taking my daughter's life."