Mannosyltransferase involved in glycosylphosphatidylinositol-anchor biosynthesis. Transfers the first alpha-1,4-mannose to GlcN-acyl-PI during GPI precursor assembly. Required for cell wall integrity (By similarity). Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. Belongs to the PIGM family.