A 16-year-old female with a multi-year history of Systemic Lupus Erythematosus presents with new-onset hypertension and lower extremity edema. A renal biopsy is performed to stage her lupus nephritis. The pathologist reports diffuse proliferative glomerulonephritis. According to the International Society of Nephrology classification, which histologic class corresponds to this finding and carries the most significant risk for renal morbidity?