Two men and two women have been arrested in London in connection with the murder of Mary-Ann Leneghan, whose family come from Co Mayo.
A Thames Valley Police spokeswoman said both men had been detained on suspicion of murder and the women on suspicion of assisting an offender.
All four are being held at police stations across London awaiting interview, she said.
Police are continuing to search a house in Tipton and other addresses in London.
The officer leading the inquiry into the murder said that "good progress" was being made.
Det Supt Mark Warwick of Thames Valley Police said earlier that three other men were being questioned in connection with the killing of 16-year-old Mary-Ann, who was sexually abused and beaten before being stabbed in the neck and dumped in a park in Reading, Berkshire.
Det Supt Warwick said: "It is now five days since Mary-Ann was brutally murdered and I'm satisfied that good progress is now being made by my team." Det Supt Warwick said Reading magistrates had given detectives a warrant to continue holding a 21-year-old man, arrested at lunchtime on Tuesday in London, until the weekend.
Det Supt Warwick said hundreds of hours of CCTV footage were being examined by forensic teams in an effort to trace the movements of a gang of up to six men who kidnapped Mary-Ann and her 18-year-old friend late on Friday night of last week, forcing them into a maroon-coloured car before driving around the Reading area playing loud music.
Police said the 18-year-old friend, who survived the attack despite being shot in the head and left for dead, was continuing to help detectives from her hospital bed.
"She has been very lucky. The bullet did not penetrate her skull and she has not suffered brain damage," a spokeswoman said.
Detectives are continuing to talk to a third girl who escaped the attackers when they grabbed the two girls from a disused pub car-park.
They are also looking into claims that Mary-Ann was seen struggling with a man at the same car-park the day before she was kidnapped.
Fresh reports that the car was seen being driven by the gang in the early hours of Saturday in villages surrounding Reading as well as the town itself are also being investigated.
Mary-Ann and her friend, who cannot be named for legal reasons, were dumped in Prospect Park in Reading.
More than 100 people attended a meeting held by Reading police on Thursday night to reassure residents following Mary-Ann's murder. - (PA)