A 22-year-old was arrested by officers at an address in Croydon on the evening of Wednesday, August 3, on suspicion of murder.
All four men remain in custody, after officers secured warrants to extend their detention.
He said: “While a further arrest has been made, this remains a missing person investigation and our priority is, and has always been, finding Owami.
“I would continue to appeal to anyone who has information about Owami’s whereabouts to please get in contact with my team.”
He added: “If you don’t feel comfortable speaking to police, you can call the independent charity Crimestoppers – they will not ask your identity and will not trace your call.”
Owami left her family home on Monday, July 4, and was last seen just after midnight in Derby Road, West Croydon, on Thursday, July 7.
Her family are being kept updated about the progress of the investigation.
Police are also appealing for the driver of a white van that was parked in Derby Road, at the time Owami was last seen, to come forward..
She was spotted in the company of a man and crossed the road just as the van pulled out.
Police said the driver would have had a good opportunity to see them both clearly and officers are appealing for this possible witness to come forward.
Det Ch Insp Penney added: “We are working hard to find Owami but we really do need the public’s help - the longer she is missing the more we worry that she has come to harm.
"As a result of extensive CCTV enquiries we have a clearer idea of her movements, but we still need the public to contact us and tell us what they know.
“We would obviously like to speak to the driver of the white van that passed Owami in Derby Road, but anyone with information should call.”
And Owami’s mother Nicol Davies said: “There are many friends and family who love and care for Owami and all of them want to see her safe return.
“Owami - if you see this appeal please believe you are not in any trouble, we just want you to come back to us.
“Please leave a message if you can to let us know you are okay, we are so very worried about you.”
And she added: "This is such a terrible time for all those who love Owami and I am pleading for anyone who can help find her to contact the police.
“Every day we hope to hear the news that Owami is coming back to us, your help might make it so.”
Any witnesses or anyone with any information is asked to call police on the incident room at 020 8721 4622. You can also call 101 or contact via Twitter @MetCC. Please quote reference 22MIS025307. To give information anonymously contact Crimestoppers on 0800 555 111 or online at crimestoppers-uk.org