The four patients were treated at the scene and taken to a nearby trauma centre, a London Ambulance Service spokesperson said.
The Metropolitan Police said four men - aged 34, 39, 42 and 52 - were found with stab wounds.
The attack happened at lunchtime in Covent Garden, the capital's busy tourist area filled with bars, theatres and restaurants.
A 47-year-old woman was arrested on suspicion of possession of an offensive weapon and assault, the force added.
She was taken to custody where she remains.
Police said they believed the stabbing was a mental health-related incident and a pair of scissors were recovered at the scene, police said.
Police previously said that three people had been stabbed in the incident but in a statement they later confirmed four people were found with stab wounds.
There was a significant police cordon on Endell Street in Covent Garden.
The cordon started at High Holborn and covers the St Mungo's homeless centre on the street, where officers were seen speaking to people before they entered the building.
Matt Bishop, 42, who works at a consultancy firm next to the cordon, said he had seen a woman having a "stand-off" with three police officers and waving a weapon around.
He added that he did not see the stabbing but saw the woman running towards the crossroads and said she seemed "pretty incoherent".
A restaurant manager said a woman "started stabbing random people" on a busy road in Covent Garden.
Arjan Gjorga, who manages a fish and chip shop in Endell Street, said the victims were "random people who were just walking by".
with PA
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