Component of the zona pellucida, an extracellular matrix surrounding oocytes which mediates sperm binding, induction of the acrosome reaction and prevents post-fertilization polyspermy. The zona pellucida is composed of 3 to 4 glycoproteins, ZP1, ZP2, ZP3, and ZP4. ZP4 may act as a sperm receptor. Expressed in oocytes. The ZP domain is involved in the polymerization of the ZP proteins to form the zona pellucida. Proteolytically cleaved before the transmembrane segment to yield the secreted ectodomain incorporated in the zona pellucida. Belongs to the ZP domain family. ZPB subfamily.