Main textThis report describes the fourth CCM key comparison in capillary viscometry among twelve National Metrology Institutes (NMIs), of which, nine NMIs maintain an independent viscosity scale and three NMIs do not maintain an independent viscosity scale. The comparison was carried out between January 8th 2018 and April 30th 2018. Two samples of Newtonian liquids with nominal kinematic viscosities of 170 mm2/s at 20°C, 65 mm2/s at 40°C and 10 mm2/s at 100°C for sample A and 10 000 mm2/s at 10°C 4 600 mm2/s at 20°C and 120 mm2/s at 100°C for sample B. Both samples were prepared by CENAM and provided to each one of the participants NMIs. For each of the samples at three temperatures, a total number of 64 measurements were carried out by the participants. From the values reported for the viscosity measurements, the key comparison reference values (KCRV) for six data sets were calculated. The degrees of equivalence were evaluated by the difference of the reported viscosity value with the KCRV. It was found that 9 measurements (14%) showed a degree of equivalence greater than the estimated uncertainty for the degree of equivalence.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).