A 22-year-old male presents with dizziness, fatigue, epistaxis, and gingival bleeding for 1 week. Laboratory tests: Hb 85g/L, WBC 42×10<sup>9</sup>/L, PLT 23×10<sup>9</sup>/L. Peripheral blood smear shows immature cells. Bone marrow smear reveals hypercellularity with 50% blasts, 21% promyelocytes, strongly positive POX, negative NAP, and NSE partially positive (not inhibited by NaF). The diagnosis is acute myeloid leukemia (AML). What is the FAB classification?