  We investigate a 4-state ferromagnetic Potts model with a special type of
geometrical frustration on a three dimensional diamond lattice by means of
Wang-Landau Monte Carlo simulation motivated by a peculiar structural phase
transition found in $\beta$-pyrochlore oxide KOs$_2$O$_6$. We find that this
model undergoes unconventional first-order phase transition; half of the spins
in the system order in a two dimensional hexagonal-sheet-like structure, while
the remaining half stay disordered. The ordered sheets and the disordered
sheets stack one after another. We obtain a fairly large residual entropy at $T
= 0$ which originates from the disordered sheets.
