  We examine the prospects for determining the neutrino mixing matrix and for
observing CP and T violation in neutrino oscillations in four-neutrino models.
We focus on a general class of four-neutrino models with two pairs of nearly
degenerate mass eigenstates separated by approximately 1 eV, which can describe
the solar, atmospheric and LSND neutrino data. We present a general
parametrization of these models and discuss in detail the determination of the
mixing parameters and the mass matrix texture from current and future neutrino
data in the case where $\nu_e$ and $\nu_\mu$ each mix primarily with one other
neutrino. We find that measurable CP/T violation in long-baseline experiments,
with amplitude at the level of the LSND signal, is possible given current
experimental constraints. Also, additional oscillation effects in short- and
long-baseline experiments may be measurable in many cases. We point out that,
given separate scales for the mass-squared differences of the solar and
atmospheric oscillations, observable CP/T violation effects in neutrino
oscillations signals the existence of a sterile neutrino. We examine several
textures of the neutrino mass matrix and determine which textures can have
measurable CP/T violation in neutrino oscillations in long-baseline
experiments. We also briefly discuss some possible origins of the neutrino mass
terms in straightforward extensions of the Standard Model.
