  We study quantum mechanics of a massive superparticle in d=4 which preserves
1/4 of the target space supersymmetry with eight supercharges, and so
corresponds to the partial breaking N=8 down to N=2. Its worldline action
contains a Wess-Zumino term, explicitly breaks d=4 Lorentz symmetry and
exhibits one complex fermionic kappa-symmetry. We perform the Hamiltonian
analysis of the model and quantize it in two different ways, with gauge-fixed
kappa-symmetry and in the Gupta-Bleuler formalism. Both approaches give rise to
the same supermultiplet structure of the space of states. It contains three
irreducible N=2 multiplets with the total number of (4+4) complex on-shell
components. These states prove to be in one-to-one correspondence with the de
Rham complex of p-forms on a three-dimensional subspace of the target
x-manifold. We analyze the vacuum structure of the model and find that the
non-trivial vacua are given by the exact harmonic one- and two-forms. Despite
the explicit breaking of the d=4 Lorentz symmetry in the fermionic sector, the
d=4 mass-shell condition is still valid in the model.
