A man, 35, was arrested on Sunday evening on suspicion of assisting an offender, the force added.
It followed the arrest of a 35-year-old man on suspicion of murder, and two men, aged 21 and 22, who were arrested on suspicion of assisting an offender.
Police and ambulance crews were called at about 05:30 BST on Saturday to a woman who had been assaulted in Chadwell Heath Lane.
Ms Mukhtar’s family said: “Zahwa’s tragic loss has left us heartbroken, and she will be truly missed by all who knew and loved her. She was dearly loved by her family and friends.”
Det Sup Brian Hobbs said: “Our thoughts today are with the victim’s family and friends in light of this truly tragic event.
“Although we have now made arrests, local residents will continue to see an increased number of officers in and around the area while our investigative work is carried out.
“I would ask anyone who was in the area of Romford, who may have seen or heard anything suspicious, to speak to us.”