It is involved in the regulation of the biosynthesis and biological function of glycoprotein oligosaccharides. Catalyzes the addition of N-acetylglucosamine in beta 1-4 linkage to the beta-linked mannose of the trimannosyl core of N-linked sugar chains, called bisecting N-acetylglucosamine (GlcNAc). It is one of the most important enzymes involved in the regulation of the biosynthesis of glycoprotein oligosaccharides. The addition of this bisecting GlcNAc residue alters not only the composition, but also the conformation of the N-glycan. The introduction of the bisecting GlcNAc residue results in the suppression of further processing and elongation of N-glycans, precluding the formation of beta-1,6 GlcNAc branching, catalyzed by MGAT5 since it is unable to use the bisected oligosaccharide as a substrate (PubMed:19403558). Addition of bisecting N-acetylglucosamine to CDH1/E-cadherin modulates CDH1 cell membrane location (PubMed:19403558). Inhibits NeuAc-alpha-2,3-Gal-beta-1,4-GlcNAc- formation which modulates sialylation levels and plays a role in cell migration regulation (PubMed:26801611). In brain, addition of bisecting N-acetylglucosamine to BACE1 blocks its lysosomal targeting in response to oxidative stress and further degradation which increases its location to early endosome and the APP cleavage (By similarity). Adds bisecting GlcNAc residue to complex-type N-linked-glycans attached on fragment crystallizable (Fc) of IgGs. Readily converts fucosylated and non-fucosylated core glycoforms with terminal GlcNAcs on both antennae. Prior galactosylation of GlcNAc on the alpha(1->3) Man branch prevents N-glycan bisection, whereas galactosylation of GlcNAc on the alpha(1->6) Man branch is permissive. {ECO:0000250|UniProtKB:Q10470, ECO:0000269|PubMed:19403558, ECO:0000269|PubMed:26801611, ECO:0000269|PubMed:27872474, ECO:0000269|PubMed:37632720}. This is the function of MGAT3 (beta-1,4-mannosyl-glycoprotein 4-beta-N-acetylglucosaminyltransferase, Ensembl gene identifier ENSG00000128268).