Mannosyltransferase involved in glycosylphosphatidylinositol-anchor biosynthesis. Transfers a fourth mannose to some trimannosyl-GPIs during GPI precursor assembly. The presence of a fourth mannose in GPI is facultative and only scarcely detected, suggesting that it only exists in some tissues. Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. Widely expressed at low level, with highest level in brain and colon. Belongs to the glycosyltransferase 22 family. PIGZ subfamily. It is uncertain whether Met-1 or Met-32 is the initiator.