Dual superfield approach to supersymmetric mechanics with spin variables

We consider a reducible ${\cal N}=4$, $d=1$ multiplet described by a real superfield as a coupling of the mirror ${\bf (1, 4, 3)}$ and ordinary ${\bf (3, 4, 1)}$ multiplets. Employing this so-called "long multiplet", we construct a coupled system of dynamical and semi-dynamical multiplets. We show that the corresponding {\it on-shell} model reproduces the model of Fedoruk, Ivanov and Lechtenfeld presented in 2012. Furthermore, there is a hidden supersymmetry acting on the long multiplet that extends the full world-line supersymmetry to SU(2$|$2). In other words, the ${\cal N}=4$ long multiplet can be derived from an irreducible SU(2$|$2) multiplet.