"The accused persons carried out a terrorist attack at Westgate Shopping Mall on September 21 by supporting a terrorist group," the charge sheet read.
All pleaded not guilty to the charges, which also included entering Kenya illegally and obtaining false identification documents.
None are accused of being the gunmen in the mall.
The four, who are all ethnic Somalis, are Mohammed Ahmed Abdi, Liban Abdullah, Adan Adan and Hussein Hassan.
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All the gunmen in the Westgate attack -- totalling just four, not the dozen that security forces had initially reported -- are understood to have died during the four-day siege.
Witnesses in the mall described how the fighters stormed the crowded complex, firing from the hip and hurling grenades at shoppers and staff.
The gunmen coldly executed scores of people, with witnesses recounting how in some cases they called out to those wounded, then shot them at close range.