  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 with
previous work, we study the redshift evolution of the rest-frame UV line
properties across the entire redshift range, 1.5<z<4.0. We improve the systemic
redshift estimates at z>3.5 using a cross-correlation algorithm that employs
high signal-to-noise template spectra spanning the full range in UV emission
line properties. We then quantify the evolution of C IV and He II emission line
properties with redshift. The increase in C IV blueshifts with cosmological
redshift can be fully explained by the higher luminosities of quasars observed
at high redshifts. We recover broadly similar trends between the He II EW and C
IV blueshift at both 1.5<z<2.65 and 3.5<z<4.0 suggesting that the blueshift
depends systematically on the spectral energy density (SED) of the quasar and
there is no evolution in the SED over the redshift range 1.5<z<4.0. C IV
blueshifts are highest when L/LEdd > 0.2 and Mbh > 10^9 Mo for the entire
1.5<z<4.0 sample. We find that luminosity matching samples as a means to
explore the evolution of their rest-frame UV emission line properties is only
viable if the samples are also matched in the Mbh - L/LEdd plane. Quasars at
z>6 are on average less massive and have higher Eddington-scaled accretion
rates than their luminosity-matched counterparts at 1.5<z<4.0, which could
explain the observed evolution in their UV line properties.
