A 21-year-old female presents with a 6-month history of low-grade fever and 2 weeks of hematuria. Urine sediment shows RBCs covering the entire field/HPF, with 80% being dysmorphic. Urinary protein (++). Rash on the face and limbs. Blood pressure (BP) 150/90 mmHg. White blood cell (WBC) count 3.0×10^9/L, plasma albumin (Alb) 3.6 g/L, serum creatinine (Scr) 80 μmol/L, immunoglobulin G (IgG) 18.2 g/L, C3 0.58 g/L, antinuclear antibody (ANA) 1:640. The clinical diagnosis is systemic lupus erythematosus (SLE). Assuming this is Class IV lupus nephritis, what is the treatment principle?