Very few segments of the world's languages have been shown to have a systematic effect on the fourth formant (F4). We investigate a large drop in F4 which sometimes occurs in conjunction with the flap in American English. The goal of the present work is to document this phenomenon, and to determine what phonological environments coincide with this large drop in F4. We measure data from six speakers producing words with medial flaps in various environments, such as party, turtle, bottle, credit, harder. We find that the combination of flap with a rhotic and to a lesser extend a syllabic /l/ leads to a larger drop in F4 than other flap combinations like a following /i/. Together with previous perceptual data, the findings support the conclusion that this feature of F4 results from transitions among articulations.