A 63-year-old female patient with a history of coronary artery disease (CAD) presents with recurrent edema and oliguria for 10 years. Laboratory findings: Hb 70g/L, BP 20/13.3 kPa, urinary protein (++), 6-10 RBCs/HPF, 2-3 WBCs/HPF, urine specific gravity 1.010-1.012. Clinical diagnosis: chronic glomerulonephritis (CGN). Blood tests: BUN 31mmol/L, Cr 407μmol/L, Ccr 40ml/min. Which renal function stage does this patient belong to?