Heterotetramer of two type I and two type II keratins. Keratin-4 is generally associated with keratin-13. Expressed in the dorsal and ventral epithelium of the tongue. Highest expression levels are detected in the suprabasal layer with low levels detected in the basal cell layer. Within the suprabasal layer expression is highest in the spinous cells, decreases in the granular cells and is not detected in the stratum corneum. There are two types of cytoskeletal and microfibrillar keratin: I (acidic; 40-55 kDa) and II (neutral to basic; 56-70 kDa). Belongs to the intermediate filament family.