Frequency effects on the performance of a two-stage 4K pulse tube cooler (PTC) have been extensively explored. The dependence of refrigeration temperature, cooling power and efficiency on the operating frequency of a 4K PTC are investigated and compared with those of G-M and Stirling coolers. The cooling performance can be considerably improved by frequency optimization. Cooling powers of 30W at 70K, 500mW at 4.2K (1.2Hz) and 20W at 65K, 590mW at 4.2K (1.1Hz) are obtained.