In this paper, the fourth in a new series, we have examined the properties associated with illuminated sectors of the annular “sensitization” zone of the psychophysical Westheimer function. The results for one-, two-, and four-sector annular background fields indicate simple additivity of effects within the sensitization region when the flashing test field was set substantially above its increment threshold. Under such conditions effects of pattern orientation and borders within the sensitization zone were found to be negligible. The present findings provide a basis for relating the newly developed “transient-like” psychophysical test (which uses a windmill-type stimulus) to the Westheimer function