Location of our sources through the optical plane of the 4DE1. Pop. A quasars are represented by blue circles and Pop. B by red circles. The radio-loud sources from the sample are surrounded by a circle in the plot. Grey and green regions indicate the distribution of the main sequence of quasars across the 4DE1 optical plane in a low-z context (Marziani et al. 2018). Spectral types (ST) of the MS are labelled according to Sulentic et al. (2002). Blue and red squares indicate high-redshift Pop. A and Pop. B quasars from M09, respectively. Purple-shaded areas show the luminosity-dependent boundary between Pops. A and B and the minimum FWHM value for sources with 47 ≤ L_bol ≤ 48 and assuming α = 0.67 (Marziani et al. 2009).