4D2N:
reference: Crystal structure of deglycosylated human IgG4-Fc., Davies AM, Jefferis R, Sutton BJ, Mol Immunol. 2014 Jun 20;62(1):46-53. doi: 10.1016/j.molimm.2014.05.015. PMID: 24956411

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).