We present a method of smoothing discrete breakup S-matrix elements calculated by the method of continuum-discretized coupled-channels (CDCC). This smoothing method makes it possible to apply CDCC to four-body breakup reactions. The reliability of the smoothing method is confirmed for two cases, Ni(d, pn) at 80 MeV and the E1 transition of He. We apply CDCC with the smoothing method to a He bre...