Heterotetramer of two type I and two type II keratins. Keratin-4 is generally associated with keratin-13. Detected in the suprabasal layer of the stratified epithelium of the esophagus, exocervix, vagina, mouth and lingual mucosa, and in cells and cell clusters in the mucosa and serous gland ducts of the esophageal submucosa (at protein level). Expressed widely in the exocervix and esophageal epithelium, with lowest levels detected in the basal cell layer. Three alleles of K4 are known: K4A1, K4A2 and K4B (displayed here). The disease is caused by variants affecting the gene represented in this entry. 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.