Excited-state enol to keto tautomerization of 7-hydroxy-4-methylcoumarin (C456) with three water molecules (C456:3H(2)O), is theoretically investigated using time-dependent density functional theory (TDDFT) combined with the polarizable continuum model and 200 waters explicitly modeled with the effective fragment potential. The tautomerization of C456 in the presence of three water molecules is accompanied by an asynchronous quadruple hydrogen atom transfer reaction from the enol to the keto tautomer in the excited state. TDDFT with the PBEO functional and the DH(d,p) basis set is used to calculate the excited-state reaction barrier height, absorption (excitation), and fluorescence (de-excitation) energies. These results are compared with the available experimental and theoretical data. In contrast to previous work, it is predicted here that the coumarin 456 system undergoes a hydrogen atom transfer, not a proton transfer. The calculated reaction barrier of the first excited state of C456:3H(2)O with 200 water molecules is found to be -0.23 kcal/mol without zero-point energy (-5.07 kcal/mol with zero point energy, i.e., the activation energy).