Keelan was stabbed more than 40 times in a "targeted, well-planned" attack yards from his Wolverhampton home on May 29 2018.
A 13-week trial at the city's Crown Court ended with unanimous guilty verdicts being returned against Brian Sasa and Nehemie Tampwo, both 20, and Tyrique King and Zenay Pennant-Phillips, 19.
All were 17 at the time of the killing apart from Tyrique King, who was 16.
Jurors heard that Keelan was stabbed with a variety of weapons in the back of a taxi in Langley Road, in the Merry Hill area of Wolverhampton.
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The attack followed rising tensions between the city's V2 and V3 postcode gangs, the court was told, with the murder the last of at least three violent clashes between the gangs on the same day.
The first involved machetes and started at a barber shop in the city centre before spilling onto the street at around 3.40pm, while the second saw youths on bikes fire shots towards Keelan in Legge Street, Parkfield.
The day of violence culminated with Keelan being knifed minutes after calling a taxi at around 11pm. By the time the killers fled, Keelan had been stabbed 43 times and dragged outside of the car.
DNA evidence placed Tampwo, Sasa and Pennant-Phillips at the scene of crime.
Tampwo was found in possession of clothing the day after containing Keelan's blood, while Sasa and Pennant-Phillips' DNA were found on the taxi.
All four killers had denied murdering Keelan and also of being part of the V3 gang.
At the start of the trial, prosecutor Michael Duck QC said: "It is a matter of real sadness that people of such ages have the willingness to take the life of another person.
"It is not just the fact they were prepared to take that life but the manner in which they chose to do it.
"This was not in any way a spontaneous act of violence.
"This was a well-planned, targeted group attack by a number of youths armed with knives, and that was with the plan to execute another young man."
Mr Duck said there was evidence to suggest the victim had "become embroiled in gang culture" despite his age.
Sasa, of Long Ley, Heath Town; King, of Chelwood Gardens, Wolverhampton; Tampwo, of Fern Grove in Bletchley, Milton Keynes, Buckinghamshire; and Pennant-Phillips, whose address cannot be published for legal reasons, will be sentenced in March.
Verdicts in the case were returned around two hours after Judge Michael Chambers QC discharged a jury member who had tested positive for coronavirus last week.
Praising the jurors for their patience and good will during what he had earlier described as "endless" Covid-related adjournments, Judge Chambers said: "It's customary at the conclusion of every trial for the trial judge to thank the jury for their service.
"In a case like this you deserve particular praise.
"It's essential that the rule of law, particular in cases such as this, should continue."
The judge said of Keelan's death: "It is an utter tragedy that a 15-year-old child lost his life at the hands of others who are barely older then he."