A 68-year-old man undergoes transrectal ultrasound-guided biopsy of the prostate for elevated PSA (8.2 ng/mL) and abnormal digital rectal examination. Histopathology reveals adenocarcinoma with the architectural pattern shown as **B** in the diagram — fused glands with cribriform structures and loss of normal glandular definition. The primary pattern is **B**, and the secondary pattern is Pattern 3 (discrete glands). According to modern ISUP 2014 grading, what is the prognostic grade group assignment and expected clinical behavior?