Mannosyltransferase involved in glycosylphosphatidylinositol-anchor biosynthesis. Transfers a fourth mannose to some trimannosyl-GPIs during GPI precursor assembly (By similarity). Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. Belongs to the glycosyltransferase 22 family. PIGZ subfamily.