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. Belongs to the glycosyltransferase 22 family. PIGZ subfamily. Extended N-terminus.