Heterotetramer of two type I and two type II keratins. Synthesis begins slightly higher in the hair shaft than HB1 and HB3 and continues much farther up, ending in the keratogeneous zone. The disease is caused by variants affecting the gene represented in this entry. There are two types of hair/microfibrillar keratin, I (acidic) and II (neutral to basic). Belongs to the intermediate filament family.