Samuel William Davidson was driving erratically and speeding when he struck Veronique Sakr, 11, and her cousins, Sienna Abdallah, eight, and her siblings Angelina, 12, and Antony, 13.
The 29-year-old ploughed into them after his ute mounted a kerb at Oatlands, in Sydney's northwest on February 1, 2020.
He was jailed in April 2021 for 28 years with a non-parole period of 21 years.
But on Friday in a majority decision, the NSW Court of Criminal Appeal concluded the term was manifestly excessive.
Davidson was resentenced to a term of 20 years with a non-parole period of 15 years.
The professional truck driver pleaded guilty to four counts of manslaughter and to charges related to the injuries caused to three other children.
One boy has suffered permanent brain damage.
The sentencing judge said Davidson's menacing, dangerous and aggressive driving showed all responsibility to the road safety of others was abandoned.
The tragedy was inevitable, but the magnitude of the tragedy extended to the unimaginable, he said.
Justice Christine Adamson dissented from the conclusion of Justices Paul Brereton and Natalie Adams who found the "aggregate" term - which is imposed instead of separate terms for each offence - was manifestly excessive.
"The sentencing judge was faced with a difficult task in sentencing the applicant for the highest number of manslaughter charges arising from the one act of criminal negligence in NSW to date," Justice Adams said.
While there were no other aggregate sentences for four deaths for the appeal court to consider, she said there were cases where the criminally negligent driving was comparable even though it resulted in fewer deaths.
After considering these cases, relevant sentencing principles and other data, she was satisfied the aggregate sentence imposed was "unreasonable or plainly unjust".
"The real question raised by this appeal is how the fact of the multiple deaths is to be punished in circumstances where the same act in driving caused four deaths," she said.
"His one act of criminally negligent driving had catastrophic consequences."
In resentencing, she said Davidson was a man of prior good character, had genuine remorse, little risk of re-offending and had suffered the long-term impact of ADHD
"As the sentencing judge noted, the purposes of specific deterrence, protection of the community and rehabilitation are of less significance in this matter."
Danny Abdullah, father of three of the children who were killed, wrote a letter saying no sentence imposed would ever be enough for the loss the families have suffered.
"But this Court is required to reconcile their grief with the application of the relevant sentencing principles and, by way of instinctive synthesis, arrive at the appropriate aggregate sentence to punish the applicant," the judge said.
The dissenting judge, Justice Adamson, said the District Court judge's carefully expressed and well-reasoned judgment explained how and why the aggregate sentence was imposed.
"There is no error in either the process or the result," she said.
The sentence imposed was lengthy because the crimes were so objectively serious, while there were limited subjective factors in his favour.
"I am not persuaded that this Court ought intervene on the basis that the sentence might be regarded by the applicant as crushing."
Mr Abdallah and his wife Leila Abdallah have previously said they had forgiven the driver.
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