R-band magnitude (top left panel), 1.4 GHz flux density (top right panel), 0.2–12 keV flux (bottom left panel), and BlaST-derived synchrotron peak frequency (bottom right panel) as a function of redshift for sources with a counterpart either in the 4XMM–DR13 catalog or in the 2CSC, with (blue squares) and without (red circles) a counterpart in the 4FGL-DR4 Fermi-LAT catalog. As a reference, we also include in the plot the 72 sources with redshift information and a counterpart in the TeVCAT catalog (cyan diamonds), which are therefore known TeV emitters. In the plot, we also include the median values of each parameter for both the 4FGL and the non-4FGL sample: we include in square parentheses the first and third quartiles of each subsample.