The competition between radiative and nonradiative recombination of electron-hole pairs in (Al,In,Ga)N/(Al,In,Ga)N quantum wells of two different compositions and various well widths was investigated in order to understand the microscopic mechanisms of the highly intense light emission from this type of structures. By using time-resolved photoluminescence, we have verified that one can adjust the compositions of the quaternaries so as to optimize the confinement effects and to minimize the built-in electric field which is present in such hexagonal group-III nitride based QWs. The role of local potential fluctuations in the quaternary alloy on the localization of carriers was studied and analyzed by measuring the photoluminescence energy, intensity and dynamics versus temperature.