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The 4D (four-dimensional) eigenvector 1 (E1) sequence has proven to be a highly effective tool for organizing observational and physical properties of type-1 active galactic nuclei (AGNs). In this paper, we present multiple measurements of metallicity for the broad line region gas, from new and 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 Balmer hydrogen <span style="font-variant: small-caps;">H</span><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>β</mi></semantics></math></inline-formula> profile 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 Fe<span style="font-variant: small-caps;">ii</span> emission, large <span style="font-variant: small-caps;">H</span><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>β</mi></semantics></math></inline-formula> FWHM (full width at half maximum)) to metallicity several tens the solar value in correspondence with extreme Population A (strongest Fe<span style="font-variant: small-caps;">ii</span> optical emission, narrower <span style="font-variant: small-caps;">H</span><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>β</mi></semantics></math></inline-formula> profiles). The data establish the metallicity as a correlate of the 4DE1/main sequence. If the considerably high metallicity (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>Z</mi><mo>≳</mo><mn>10</mn><msub><mi>Z</mi><mo>⊙</mo></msub></mrow></semantics></math></inline-formula>, solar metallicity) gas 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 a high Eddington ratio, then it is possible that the outflows from quasars played a role in chemically enriching the host galaxy.