2HK4:
reference: Four-stranded DNA structures can be stabilized by two different types of minor groove G:C:G:C tetrads., Escaja N, Gomez-Pinto I, Pedroso E, Gonzalez C, J Am Chem Soc. 2007 Feb 21;129(7):2004-14. Epub 2007 Jan 30. PMID: 17260988

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