Nadeem Ilyas Patel, 25, from east London, has been charged for possession of a firearm with intent to cause fear of violence.
Patel appeared at Westminster Magistrates' Court and has been remanded in custody along with Umar Ahmed Haque, Muhammed Abid and Abuthater Mamun.
All four will stand trial at the Old Bailey Court in London from June 9.
"Nadeem Ilyas Patel, from Newham was charged with an offence contrary to Section 1(1) of the Criminal Law Act 1977, on 30 March 2017 conspired together with Umar Haque, to possess any firearm or imitation firearm with intent to cause fear of violence," the Metropolitan Police said in a statement.
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Haque, 24, is accused of planning an attack involving mass murder using a vehicle, knives or firearms.
Abid, 27, has been charged under terror laws with failing to disclose information and 18-year-old Mamun is accused of assisting another to commit acts of terrorism.
All three have been charged under the UK's Terrorism Act.
Patel is accused of conspiring with Haque as part of what the Met Police suspect to be a dangerous terrorist plot.