  We present a systematic study of twist-4 light-cone distribution amplitudes
of vector mesons in QCD, which is based on conformal expansion. The structure
of meson mass corrections is studied in detail. A complete set of distribution
amplitudes is constructed, which satisfies all (exact) equations of motion and
constraints from conformal expansion. Nonperturbative input parameters are
estimated from QCD sum rules. Our study suggests that meson mass corrections
may present a dominant source of higher twist effects in exclusive processes.
