Beta-mannosyltransferase involved in cell wall biosynthesis. Responsible for addition of a hexose to the beta-mannose chain. Beta-mannosylation catalyzed by BMT4 masks beta-mannose epitopes recognized by monoclonal antibody (mAb) 5B2 reacting with beta-1,2-linked mannobiose as a minimal epitope. Leads to a size reduction of glycans resistant to alpha-mannosidase treatment. Belongs to the BMT family. Truncated N-terminus.