"This is the fourth report of a series to be compiled on operations on the N-T-U retorts. Report No. 1 described the N-T-U retort process and discussed preliminary operations. Reports No. 2 and No. 3 covered operations using recycle gas instrument settings of 200 and 300 cfm respectively, with the air rate being varied over the range of the air blower capacity. This report covers operations using a recycle gas instrument setting of 400 cfm and varying the air rate over the range of the air blower capacity. All other operating conditions were held as close as possible to those used during the 200 and 300 cfm recycle gas series. Forty-two tons of minus 31/2-inch plus 1/2 inch ""G"" bed shale assaying approximately 30 gallons per ton were charged for each run, and operating techniques were held constant. All test runs for this series were completed between May 27, 1948, and October 26, 1948 . A total of 714 tons of oil shale was charged, and 17,460 gallons of crude shale oil were recovered for the seventeen runs. The average recovery was 71.6 percent based on the Fischer assay, and the average recovery rate was 47.6 gallons of oil per hour for each of the 367.1 operating hours. The average Fischer assay was 32.2 gallons per ton."