Catalyzes alpha(1->3) linkage of fucosyl moiety transferred from GDP-beta-L-fucose to N-acetyl glucosamine (GlcNAc) within type 2 lactosamine (LacNAc, Gal-beta(1->4)GlcNAc) glycan attached to N- or O-linked glycoproteins (PubMed:11485743). Robustly fucosylates nonsialylated distal LacNAc unit of the polylactosamine chain to form Lewis X antigen (CD15), a glycan determinant known to mediate important cellular functions in development and immunity. Fucosylates with lower efficiency sialylated LacNAc acceptors to form sialyl Lewis X and 6-sulfo sialyl Lewis X determinants that serve as recognition epitopes for C-type lectins (PubMed:11485743). Together with FUT7 contributes to SELE, SELL and SELP selectin ligand biosynthesis and selectin-dependent lymphocyte homing, leukocyte migration and blood leukocyte homeostasis (PubMed:11485743). In a cell type specific manner, may also fucosylate the internal LacNAc unit of the polylactosamine chain to form VIM-2 antigen that serves as recognition epitope for SELE (By similarity). a beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + GDP-beta-L-fucose = a beta-D-galactosyl-(1->4)-[alpha-L-fucosyl-(1->3)]-N-acetyl-beta-D-glucosaminyl derivative + GDP + H(+) an N-acetyl-alpha-neuraminyl-(2->3)-beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + GDP-beta-L-fucose = an alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->4)-[alpha-L-Fuc-(1->3)]-beta-D-GlcNAc derivative + GDP + H(+) an alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->4)-beta-D-GlcNAc-(1->3)-beta-D-Gal-(1->4)-beta-D-GlcNAc derivative + GDP-beta-L-fucose = an alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->4)-beta-D-GlcNAc-(1->3)-beta-D-Gal-(1->4)-[alpha-L-Fuc-(1->3)]-beta-D-GlcNAc derivative + GDP + H(+) an alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->4)-beta-D-GlcNAc6S derivative + GDP-beta-L-fucose = an alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->4)-[alpha-L-Fuc-(1->3)]-beta-D-GlcNAc6S derivative + GDP + H(+) Protein modification; protein glycosylation. Membrane-bound form in trans cisternae of Golgi. Highest expression in stomach and colon. It is also expressed in the lung, testis, uterus, small intestine and to a lesser extent in spleen, and ovary. Present in trace amounts in brain, thymus, heart, smooth muscle, kidney and bone marrow. Not found in liver, salivary gland and pancreas. Mice are born at the expected Mendelian rate. No visible phenotype; likely due to the redundancy with FUT7. Simultaneous knockdown of FUT4 and FUT7 results in leukocytosis characterized by an 18.4 fold increase in blood neutrophils and significant increases in blood monocytes, eosinophils, and lymphocytes numbers. Belongs to the glycosyltransferase 10 family. Fucosyltransferase 4