  The vortex correlation along the c-axis in high quality single crystals of
YBa$_{2}$Cu$_{4}$O$_{8}$ has been investigated as a function of temperature T
in different magnetic fields, using the quasi-flux transformer configuration. A
simultaneous sharp drop associated with the vortex lattice melting is observed
in both the primary and secondary voltages(V$_{top}$ and V$_{bot}$). Just above
the melting temperature, the vortices form three-dimensional line liquid with
the correlation length along the c direction $L_{c}\leq $ t, the sample
thickness. The temperature where a resistive peak in R$_{bot}$ develops
corresponds to the decoupling temperature T$_{d}$ at which the vortices loose
their correlation along the c-direction and they dissolve into the two
dimensional pancake vortices. The H-T phase diagram for the
YBa$_{2}$Cu$_{4}$O$_{8}$ single crystal is obtained.
