  We present predictions for CP violating phases of the Type-I seesaw model
with four-zero textures on the Dirac mass matrix. For the four-zero textures,
the effective low energy Majorana mass matrix is parametrized with seven
parameters. They are three mass-dimensional parameters, two angles and two CP
violating sources. The number of these parameters is less than that of the
general description of the Majorana mass matrix with three neutrino masses,
three mixing angles and three CP violating phases. In particular, only two
independent CP violating sources give rise to three CP violating phases. The
efficient and comprehensive method is proposed in this paper to investigate
four-zero textures in Type-I seesaw. We numerically show the possible range of
CP violating phases in the plane of a Dirac CP violating phase and one of
Majorana phases. Some cases show the strong correlations among two phases.
These correlations can be explained by hidden relations among the elements of
Majorana matrix with the four-zero textures. The hidden relations are
classified according to the position of one vanishing off-diagonal element of
Majorana mass matrix. The Majorana mass matrix all of whose elements are
non-vanishing also produces other hidden relations particularly in the case of
four-zero textures. By applying the hidden relations, we describe the concrete
correlations among CP violating phases.
