In an effort to develop next generation UV frequency conversion systems, several steps have to be considered. One aspect crucial for the final conversion stage is a durable coating which shows high resistance for all incident wavelengths. In the regular case, two wavelengths are involved in the generation of the fourth harmonic of the Nd:YAG laser. For a conversion process involving the wavelengths 532nm and 266nm, model AR-coating designs have been developed and tested including SiO2, Al2O3, and HfO2 deposited in an IBS process. During the testing, procedures have been applied that involve both wavelengths at the same time. As in the application, the exit surface is exposed to visible and UV laser radiation, a qualifying test needs to account for these conditions as well