A 200-s PSA cycle involving both pressure equalization and product backfill steps has been experimentally studied on a four-bed system, where LINDE 5A zeolites were used as the adsorbent to separate oxygen from air. This cycle is operated under a pressure ratio of 4.3. During the experiment, the pressure history and flow rates, as well as the concentration of the product stream have been continuously monitored. This is the first time detailed experimental data on a four-bed system are presented. Under favorable conditions, this system produces better than 90% oxygen at a recovery of 17%. For the low-pressure ratio, such a recovery could not have been achieved without the pressure equalization step and the reduced purge operation. Recovery and throughput, however, are not as high as one would expect from a linear local equilibrium model. The self-broadening effect of the purge wave has been identified as the major cause of underperformance.