The boys sustained stab wounds and were taken to two hospitals – Southmead Hospital and Bristol Royal Hospital for Children – by ambulance, where they died in the early hours of Sunday morning.
Avon and Somerset Police said a 26-year-old man was arrested overnight on suspicion of assisting an offender and is currently in police custody.
Men aged 44, 22 and 20 – who were all arrested on suspicion of murder – remain in custody, while a 15-year-old also arrested on suspicion of murder has been released on bail.
Superintendent Mark Runacres, the Bristol policing commander, said: “The team investigating Mason Rist’s and Max Dixon’s deaths are working around the clock to bring those responsible to justice.
“While they are only three days into the inquiry, they have already completed a huge amount of work and the inquiry is progressing rapidly.
“Already 253 exhibits have been seized, including weapons, while they have also searched several properties.”
He added that a dedicated team is reviewing hundreds of hours of CCTV while specialist officers from neighbouring forces have been brought in to assist in order to ensure “no stone is left unturned”.
A resident who was first on the scene after the brutal attack told how Mason had begged her for help as he lay fatally injured in her former partner’s arms. Kirsty Kidd told The Independent: “We tried everything, I just wanted them to be ok.”
She added: “It could have been anyone including my own children, that’s the scary part.”
Police have previously said they did not believe the stabbings were rooted in a “gang war”.
Detectives do not yet know whether the suspects and victims were known to each other, or whether the boys died in a case of mistaken identity.
A vigil was held at the scene of the stabbings on Sunday evening, with people lighting candles and placing flowers.
Mason’s devastated grandmother visited the scene of the stabbing, where flowers, cards and candles have been left for the teenagers
Max’s heartbroken sister Kayleigh also posted an emotional tribute on Facebook on Sunday, which said: “My baby brother… one of a kind you are.
“You sleep tight. A beautiful, kind soul gone. Just taken 16 years so young and innocent oh my heart is broken.
“I really hope you know how much we love you. How much I love you. You will be missed kiddo I’ll always look out for you in every sunset, shine bright lil’ bro.”
Avon and Somerset Police have opened a murder investigation into the incident and are “keeping an open mind” as to what motivated the “senseless” attack.
Police said it is also aware there is lots of speculation, including online, about what took place.
Supt Runacres added: “Everyone across the city has been shocked by what happened to Mason and Max and our commitment to holding those responsible to account will not waiver.
“Family liaison officers continue to support Mason’s and Max’s family and are keeping them updated with developments.
“There’s been outpouring grief but also love and I know both families are really appreciative of the kindness shown to them at this difficult time.”