G4-DNA molecules were recently reported as candidates for molecular electronics because of their higher stiffness and polarizability with respect to double-stranded DNA. Short G4-DNA structures have been traditionally studied in the past because of their possible presence in the human genome. The electronic structure of these molecules, unraveled in terms of the electron energy levels, constitutes crucial information in both technology and biology contexts. The discrete electron energy-level spectrum of isolated long single G4-DNA molecules, deposited on a gold substrate, is resolved by using cryogenic scanning tunneling spectroscopy in ultra-high-vacuum conditions. The patterns of the current-voltage curves manifest a considerable, although unexpected, variability, which required the development and implementation of a dataclustering methodology in order to resolve the reproducible curve patterns. On the basis of the sorted curves, we present a statistical analysis of the electronic levels and of the width of the fundamental gap.