  The 4D Eigenvector 1 sequence has proven to be a highly effective tool for
organizing observational and physical properties of type 1 active galactic
nuclei (AGN). In this paper, we present multiple measurements of metallicity
for the broad line region gas, from new or previously published data. We
demonstrate a consistent trend along the optical plane of the E1 (also known as
the quasar main sequence), defined by the line width of H$\beta$ and by a
parameter measuring the prominence of singly-ionized iron emission. The trend
involves an increase from sub-solar metallicity in correspondence with extreme
Population B (weak FeII emission, large H$\beta$ FWHM) to metallicity several
tens the solar value in correspondence with extreme Population A (very strong
FeII optical emission, narrower H$\beta$ profiles). The data establish the
metallicity as a correlate of the 4D E1/main sequence. If the very high
metallicity gas ($Z \gtrsim 10 Z_\odot$) is expelled from the sphere of
influence of the central black hole, as indicated by the widespread evidence of
nuclear outflows and disk wind in the case of sources radiating at high
Eddington ratio, then it is possible that the outflows from quasars played a
role in chemically enriching the host galaxy.
