  The 4D eigenvector 1 (E1) sequence has emerged as a powerful tool for
organizing the observational and physical characteristics of type-1 active
galactic nuclei (AGNs). In this study, we present a comprehensive analysis of
the metallicity of the broad line region gas, incorporating both new data and
previously published findings, to assess the presence of any trend along the
sequence. We perform a multi-component analysis on the strongest UV and optical
emission lines, compute $\sim 10$ diagnostic ratios, and compare them with the
prediction of CLOUDY photoionization simulations, identifying a photoionization
solution closest to the data. Our investigation reveals a consistent pattern
along the optical plane of the E1. We observe a systematic progression in
metallicity, ranging from sub-solar values to metallicity levels several times
higher than solar values. These findings underscore the role of metallicity as
a fundamental correlate of the 4DE1/main sequence. Extreme values of
metallicity, at least several tens solar, are confirmed in low-$z$ AGNs
radiating at a high Eddington ratio, although the origin of the extreme
enrichment remains open to debate.
