Nearly identical to the M2s, Tokyu Car initially gave the "Triplex" brand name to the M4 cars to highlight their being a three-car set, as opposed to the married pairs of the M2s.
Tokyu Car's use of the "Triplex" brand name for their M4 cars emphasized that they were designed as a three-car set, contrasting them with the traditional two-car pairings found in the M2 series.