Another young man stabbed on New Year's Day is in a critical condition in hospital.
Five men have been arrested over the death of an 18-year-old in Larmans Road, Enfield. No other arrests have been made.
In the other attacks, a 20-year-old man was fatally stabbed in Memorial Avenue, West Ham, and a 17-year-old boy was killed in Norwood Road, Tulse Hill, on New Year's Eve.
Early on New Year's Day, a 20-year-old man was fatally wounded in Bartholomew Court, Old Street. A second man in his 20s suffered critical stab injuries in the same incident.
The fatalities in the final hours of 2017 took the number of people stabbed to death in the capital last year to 80, the Met said.
'Tragic consequences'
Commander Neil Jerome said: "I am grateful to Londoners and visitors to the capital who co-operated with police to enable the vast majority of people to enjoy New Year's Eve.
"However, there were small groups who chose to engage in disorder and violence, and whose actions have had utterly tragic consequences.
"It is heartbreaking that, at a time when so many of us are contemplating what lies ahead in 2018, four families are dealing with the grief of losing a loved one to senseless violence and the callous use of knives as lethal weapons."
The Mayor of London, Sadiq Khan, said: "My thoughts today are with the families of four young men who have tragically been killed in four unrelated stabbings in our city last night.
"We will work tirelessly in 2018 and beyond to stamp out this scourge. I want to be absolutely clear - if you use a knife the full force of the law will be brought down on you.
"You will be caught and prosecuted."
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