Alpha-1,2-mannosyltransferase involved in glycosylphosphatidylinositol-anchor biosynthesis. Transfers a fourth mannose to trimannosyl-GPIs during GPI precursor assembly. The presence of a fourth mannose in GPI is essential in fungi (By similarity). Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. May be used as a target for the development of some new fungicide, due the fact the presence of a fourth mannose in GPI-anchor proteins is essential for viability in fungi but not in mammals. Belongs to the glycosyltransferase 22 family. PIGZ subfamily.