Our experimental data obtained at the single-molecule level provides a detailed description of G4 folding as a complex multi-pathway process that involves numerous parallel folding/unfolding transitions and an extensive redistribution between several conformational states. Different site-specific DNA modifications can affect the energetics of the folded states and modify the conformational landscape (Figure 2). Our data show that these modified sequences exhibit similar complex folding behavior, as the one described for the wild-type sequence. The environment and specificity of folding conditions can also influence the G4 folding process. At higher potassium concentrations (100 mM KCl) similar folding behavior and conformational heterogeneity of G4s is observed (Supplementary Figure S9). Under these conditions, the folding of G4s is accelerated, compared to low salt conditions, as well as the marginally stable chair, 2-tetrad and hybrid 2 conformations get more destabilized. Overall, high potassium concentrations favor the formation of thermodynamically stable hybrid 1 conformation and result in reduction of the observed dynamics (Supplementary Figure S9). Studies in sodium-containing solutions show that G4 folding can be very dynamic and follows a multi-pathway process also in these conditions (30,58). These independent observations obtained for different DNA sequences or solutions conditions may point towards common mechanistic features of G4 folding, as described in this work.