  The $c$-axis resistivity measurements in YBa_2Cu_4O_8 from Hussey et al. for
magnetic field orientations along the c-axis as well as within the ab-plane are
analyzed and interpreted using the scaling theory for static and dynamic
classical critical phenomena. We identify a superconductor to normal conductor
transition for both field orientations as well as a normal conductor to
insulator transition at a critical field H_c||a with dynamical critical
exponent z=1, leading to a multicritical point where superconducting, normal
conducting and insulating phases coexist.
