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IRAM 30m observations reveal that the deeply obscured IR-luminous galaxyNGC4418 has a rich molecular chemistry - including unusually luminous HC3N lineemission. We furthermore detect: ortho-H2CO 2-1, 3-2; CN 1-0, 2-1; HCO+, 1-0.3-2, HCN 3-2, HNC 1-0, 3-2 (and tentatively OCS 12-11). The HCN, HCO+, H2CO andCN line emission can be fitted to densities of n=5 x 10E4 - 10E5 cm-3 and gastemperatures Tk=80-150 K. Both HNC and HC3N are, however, significantly moreexcited than the other species which requires higher gas densities - orradiative excitation through e.g. mid-IR pumping. The HCN line intensity isfainter than that of HCO+ and HNC for the 3-2 transition, in contrast toprevious findings for the 1-0 lines where the HCN emission is the mostluminous. We tentatively suggest that the observed molecular line emission isconsistent with a young starburst, where the emission can be understood asemerging from dense, warm gas with an additional PDR component. We find thatX-ray chemistry is not required to explain the observed mm line emission,including the HCN/HCO+ 1-0 and 3-2 line ratios. The luminous HC3N line emissionis an expected signature of dense, starforming gas. A deeply buried AGN can notbe excluded, but its impact on the surrounding molecular medium is thensuggested to be limited. However, detailed modelling of HC3N abundances inX-ray dominated regions (XDRs) should be carried out. The possibility ofradiative excitation should also be further investigated

Luminous HC3N line emission in NGC4418 - buried AGN or nascent starburst?