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We analyse the rest-frame UV spectra of 2,531 high-redshift (3.5<z<4.0)quasars from the Sloan Digital Sky Survey DR16Q catalogue. In combination withprevious work, we study the redshift evolution of the rest-frame UV lineproperties across the entire redshift range, 1.5<z<4.0. We improve the systemicredshift estimates at z>3.5 using a cross-correlation algorithm that employshigh signal-to-noise template spectra spanning the full range in UV emissionline properties. We then quantify the evolution of C IV and He II emission lineproperties with redshift. The increase in C IV blueshifts with cosmologicalredshift can be fully explained by the higher luminosities of quasars observedat high redshifts. We recover broadly similar trends between the He II EW and CIV blueshift at both 1.5<z<2.65 and 3.5<z<4.0 suggesting that the blueshiftdepends systematically on the spectral energy density (SED) of the quasar andthere is no evolution in the SED over the redshift range 1.5<z<4.0. C IVblueshifts are highest when L/LEdd > 0.2 and Mbh > 10^9 Mo for the entire1.5<z<4.0 sample. We find that luminosity matching samples as a means toexplore the evolution of their rest-frame UV emission line properties is onlyviable if the samples are also matched in the Mbh - L/LEdd plane. Quasars atz>6 are on average less massive and have higher Eddington-scaled accretionrates than their luminosity-matched counterparts at 1.5<z<4.0, which couldexplain the observed evolution in their UV line properties.

No redshift evolution in the rest-frame UV emission line properties of quasars from z=1.5 to z=4.0