O-linked mannose beta-1,4-N-acetylglucosaminyltransferase that transfers UDP-N-acetyl-D-glucosamine to the 4-position of the mannose to generate N-acetyl-D-glucosamine-beta-1,4-O-D-mannosylprotein. Involved in the biosynthesis of the phosphorylated O-mannosyl trisaccharide (N-acetylgalactosamine-beta-3-N-acetylglucosamine-beta-4-(phosphate-6-)mannose), a carbohydrate structure present in alpha-dystroglycan (DAG1), which is required for binding laminin G-like domain-containing extracellular proteins with high affinity (By similarity). 3-O-(alpha-D-mannosyl)-L-threonyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = 3-O-(N-acetyl-beta-D-glucosaminyl-(1->4)-alpha-D-mannosyl)-L-threonyl-[protein] + H(+) + UDP Protein modification; protein glycosylation. Expressed in the embryo and larva throughout development, with highest levels in the developing brain, eyes and at the boundaries between somites. Belongs to the glycosyltransferase 61 family.