UniProt Summary: Responsible for the synthesis of the sequence NeuAc- alpha-2,3-Gal-beta-1,3-GalNAc- found on sugar chains O-linked to Thr or Ser and also as a terminal sequence on certain gangliosides. SIAT4A and SIAT4B sialylate the same acceptor substrates but exhibit different Km values. {ECO:0000250|UniProtKB:P54751}.