The victim of a stabbing in Northolt has been named locally as Abdullahi Tarabi.
The Metropolitan Police was called to Newnham Close at 3.20pm on Tuesday (April 11) where officers found a 19-year-old male with stab wounds.
He was rushed to hospital but was pronounced dead at 5.35pm later that day.
On Wednesday (April 12) the victim was named locally as Abdullahi Tarabi, also appearing to be known on social media as Maino Tarabi.
One post on social media said: "It's mad how Tarabi was the smallest boy in the school, but with the biggest voice and personality.
"It really does hurt."
Another tribute said the victim attended West London Academy, which changed its name to Alec Reed Academy in 2012.
The post said: "Most shocking news of my 2017 so far.
"I just don't even know what to say.
"Cannot believe Maino Tarabi, aka Little Tarabi, died today after being stabbed.
"Such a popular kid at West London Academy, had so much memories with this guy and now his young life has been taken."
His next of kin have been informed.
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Formal identification awaits and a post-mortem examination will be held in due course.
An investigation is being carried out by the Homicide and Major Crime Command unit of the Met Police
Four males - three 19-year-olds and an 18-year-old - have been arrested on suspicion of murder and taken into custody at separate London police stations.
Detectives from the Homicide and Major Crime Command, led by Detective Chief Inspector Andy Partridge are investigating.
DCI Partridge said: "This deplorable attack took place in broad daylight in a residential location.
"I would like to hear from anyone who saw the attack itself or witnessed anything else that may be important.
"Whilst arrests have been made we are keen to trace a man who fled the scene in the direction of Petts Hill."
Anyone who witnessed the incident or has information about the attack is asked to contact the incident room on 0208 358 0300 or via Twitter @MetCC.
Alternatively Crimestoppers can be contacted anonymously on 0800 555 111 or online at crimestoppers-uk.org.
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