We report the characterization of metastable hydrogen-related defects (M3/M4) in n-GaAs by isothermal constant-capacitance-voltage transient spectroscopy (CCVTS). The double-correlation CCVTS spectra measured around 140 K clearly shows that the M4 defect consists of two discrete components labeled M4(1) and M4(2), which gives definitive support to the recent report that the M4 defect consists of two different configurations. The electric field dependence of the emission rates indicates that the M3 and M4(1) have a donor-like nature, while the M4(2) is acceptor like.