In this paper, we report on the experimental realization of dimensional crossover phenomena in the chain compound PrBa2Cu4O8 using temperature, high magnetic fields and disorder as independent tuning parameters. In pure crystals of PrBa2Cu4O8, a highly anisotropic three-dimensional Fermi-liquid state develops at low temperatures. This metallic state is extremely susceptible to disorder, however, and localization rapidly sets in. We show, through quantitative comparison of the relevant energy scales, that this metal/insulator crossover occurs precisely when the scattering rate within the chain exceeds the interchain hopping rate(s), i.e. once carriers become confined to a single conducting element.