Three 16-year-olds and a 15-year-old were on Monday taken into custody in Ipswich west of Brisbane and are assisting police.
Another 15-year-old boy was charged earlier and appeared in an Ipswich court on Monday following the fatal stabbing of the grandmother, identified as Vyleen White.
Ms White had been shopping with her six-year-old granddaughter at Redbank Plains west of Brisbane when she was stabbed in the chest in the centre's car park on Saturday.
She died at the scene a short time later.
The girl was unharmed.
The 15-year-old who was charged had a closed hearing in court on Monday after the magistrate refused permission for media to publish the details.
He was charged with unlawful use of a motor vehicle after handing himself into police on Sunday night.
"That does not mean there will not be further charges," Detective Acting Superintendent Heath McQueen told reporters on Monday.
Detectives had earlier released CCTV footage of four youths in a Hyundai Getz, allegedly stolen from Ms White.
"Sadly the motive was to steal a Hyundai Getz," Det McQueen said of the fatal stabbing.
Police believe the motive for the attack was to steal Ms White's car. (Jono Searle/AAP PHOTOS)
"I stand before you today talking to you about a murder of a grandmother in front of a six-year-old ... and the result was a stolen Hyundai Getz - that is the motive."
He described it as a "callous, cowardly, violent crime".
"She was a loving, family woman," he said.
"She was taking her granddaughter to the shops. This is senseless violence."
Queensland Police Commissioner Katarina Carroll said she would speak with Ms White's family on Monday.
"I offer my sincere condolences and sympathy to Vyleen's family on what has truly been a tragic day in Redlands," she said.
"We have thrown everything at this and we will continue to do so until we have identified all of the offenders and we bring them before the courts.
"We will hunt these people down until we bring them to justice."
Floral tributes for Ms White were left at the Redbank Plains shopping centre on Monday while supporters organised a GoFundMe page.
It had raised almost $13,000 for Ms White's family by Monday afternoon.
"They have lost a mother, grandmother, a wife and loved family member of their family," the fundraiser page said.
Queensland Police remain at the scene of Saturday's attack on Monday. (Jono Searle/AAP PHOTOS)
"They are all so overwhelmed and so warm hearted by the community at such a difficult time."
Fundraiser Katherine Lameree said she hoped to raise money for Ms White's family and also tell the community about her legacy.
"She was such a selfless woman - with so much care for her family and absolutely anyone she came into contact with," the page said.
"This is what they want her to be remembered by and not defined by the act of someone else."
Ipswich Mayor Theresa Harding on Monday said the attack had shocked the community, describing it as a "punch in the guts" to locals.
Ipswich Superintendent Kylie Rigg said a mobile police beat van would remain outside Redbank Plains shopping centre for weeks to reassure community safety.
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