  The recent emergence of van der Waals (vdW) ferromagnets has opened new
opportunities for designing spintronic devices. We theoretically investigate
the coherent spin-dependent transport properties of the vdW ferromagnet
Fe$_4$GeTe$_2$, by using density functional theory combined with the
non-equilibrium Green's functions method. We find that the conductance in the
direction perpendicular to the layers is half-metallic, namely it is entirely
spin-polarized, as a result of the material's electronic structure. This
characteristic persists from bulk to single layer, even under significant bias
voltages, and it is little affected by spin-orbit coupling and electron
correlation. Motivated by this observation, we then investigate the tunnel
magnetoresistance (TMR) effect in an magnetic tunnel junction, which comprises
two Fe$_4$GeTe$_2$ layers separated by the vdW gap acting as insulating
barrier. We predict a TMR ratio of almost 500\%, which can be further boosted
by increasing the number of Fe$_4$GeTe$_2$ layers in the junction.
