Golgi-resident glycosylphosphatidylinositol (GPI)-N- acetylgalactosamine transferase that catalyzes the N-acetyl-beta-D- galactosamine transfer from an UDP-N-acetyl-alpha-D-galactosamine to the 4-OH-position of the first mannose of the glycosylphosphatidylinositol (GPI) of a GPI-anchored protein (GPI-AP) (PubMed:29374258). This modification occurs after the fatty acid remodeling step of the GPI-anchor maturation (PubMed:29374258). . This is the function of PGAP4 (post-GPI attachment to proteins GalNAc transferase 4, Ensembl gene identifier ENSG00000165152).