  We present reversible magnetization data of a YBa2Cu4O8 single crystal and
analyze the evidence for 3D-xy critical behavior and a magnetic field induced
3D to 1D crossover. Remarkable consistency with these phenomena is observed in
agreement with a magnetic field induced finite size effect, whereupon the
correlation length transverse to the applied magnetic field cannot grow beyond
the limiting magnetic length scale L_H. By applying the appropriate scaling
form we obtain the zero-field critical temperature, the 3D to 1D crossover, the
vortex melting line and the universal ratios of the related scaling variables.
Accordingly there is no continuous phase transition in the (H,T)-plane along
the H_c2-lines as predicted by the mean-field treatment.
