Abstract We present the C II 158 μ m line luminosity functions (LFs) at z  ∼ 4–6 using the ALMA observations of 118 sources, which are selected to have UV luminosity M 1500Å  < −20.2 and optical spectroscopic redshifts in COSMOS and ECDF-S. Of the 118 targets, 75 have significant C II detections and 43 are upper limits. This is by far the largest sample of C II detections, which allows us to set constraints on the volume density of C II emitters at z  ∼ 4–6. But because this is a UV-selected sample, we are missing C II-bright but UV-faint sources, making our constraints strict lower limits. Our derived LFs are statistically consistent with the z  ∼ 0 C II LF at 10 8.25 –10 9.75 L ⊙ . We compare our results with the upper limits of the C II LF derived from serendipitous sources in the ALPINE maps. We also infer the C II LFs based on published far-IR and CO LFs at z  ∼ 4–6. Combining our robust lower limits with these additional estimates, we set further constraints on the true number density of C II emitters at z  ∼ 4–6. These additional LF estimates are largely above our LF at L CII  > 10 9 L ⊙ , suggesting that UV-faint but C II-bright sources likely make significant contributions to the C II emitter volume density. When we include all the LF estimates, we find that available model predictions underestimate the number densities of C II emitters at z  ∼ 4–6. Finally, we set a constraint on the molecular gas mass density at z  ∼ 4–6, with ρ mol  ∼ (2–7) × 10 7 M ⊙ Mpc −3 . This is broadly consistent with previous studies.