A hydrophilic cyclic tetra-β-peptide having guanine at the side chain (CPG) was synthesized to construct a four-peptide-nanotube bundle with the formation of guanine quadruplex (G-quartet). CPG was so soluble in water that molecular association of CPG was very limited in water. However, in the presence of potassium ion, circular dichroism spectral change showed initial molecular association due to the G-quartet formation followed by a slow growth into peptide nanotubes. Transmission electron microscope observation and electron diffraction analysis revealed that CPG self-assembled into a four-peptide-nanotube bundle with a uniform diameter of 7–8 nm. © 2012 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 100: 141–147, 2013.