  We present a study of six far-infrared fine structure lines in the z=4.225
lensed dusty star-forming galaxy SPT0418-47 to probe the physical conditions of
its InterStellar Medium (ISM). In particular, we report Atacama Pathfinder
EXperiment (APEX) detections of the [OI]145um and [OIII]88um lines and Atacama
Compact Array (ACA) detections of the [NII]122 and 205um lines. The [OI]145um /
[CII]158um line ratio is ~5x higher compared to the average of local galaxies.
We interpret this as evidence that the ISM is dominated by photo-dissociation
regions with high gas densities. The line ratios, and in particular those of
[OIII]88um and [NII]122um imply that the ISM in SPT0418-47 is already
chemically enriched close to solar metallicity. While the strong gravitational
amplification was required to detect these lines with APEX, larger samples can
be observed with the Atacama Large Millimeter/submillimeter Array (ALMA), and
should allow to determine if the observed dense, solar metallicity ISM is
common among these highly star-forming galaxies.
