L-4 was a simulated MIS experiment carried out in the 6-metric ton oil shale retort on October 22 to 24, 1979. An oil yield of 87% FA was achieved at a retorting rate of 2.8 to 3.0 m/day with a shale bed of -30 0.001 cm particle size range and including a nearly two-fold step change in grade. Temperature control was maintained by varying the steam/air ratio and injection gas rate. Oil yield, front position and retorting rate, as predicted in real time during the run from offgas data, agreed reasonably well with directly measured counterparts. Pre-run model calculations reliably predicted operating conditions for both inert gas pre-ignition and combustion retorting phases of the run. Pre-run steam logging predicted the degree of horizontal uniformity of the retorting front. Ultrasonic reflectometry, gas-releasing capsules and differential pressure measurements were shown to be promising methods for temperature measurement. Passage of the retorting front through the grade change was signalled by changes in offgas composition.