  Interlayer excitons are bound states of electrons and holes confined in
separate two-dimensional layers. Due to their repulsive dipolar interaction,
interlayer excitons can form a correlated liquid. If another electron-hole
bilayer is present, excitons from different bilayers can exhibit mutual
attraction. We study such a quadrilayer system by a hypernetted chain
formalism. We compute ground state energies, pair correlation functions, and
collective mode velocities as functions of the exciton densities. We estimate
the critical density for the transition to a paired biexciton phase. For a
strongly unbalanced (unequal density) system, the excitons in the more dilute
bilayer behave as polarons. We compute energies and effective masses of such
exciton-polarons.
