CCAST, World Lab, Beijing 100080, Peoples R China
; E China Normal Univ, Dept Phys, Shanghai 200062, Peoples R China
; Chinese Acad Sci, Inst High Energy Phys, Beijing 100039, Peoples R China
A consistent and intuitive description of the twist-4 corrections to the hadron structure functions is presented in a QCD-improved parton model using time-ordered perturbative theory, where the collinear singularities are naturally eliminated. We identify the special propagators with the backward propagators of partons in time order. [S0556-2821(99)07117-9].