This study reports the onset of the Jahn–Teller distortion in 4V LiMn2O4 thin film electrodes that was investigated using an in situ bending beam method (BBM). The phase transformation during lithium insertion/extraction could be detected using the BBM technique. The phase transformation between the cubic and tetragonal phases was depicted by the larger value of the compressive or tensile differential strain, which is consistent with a well-known phase transformation between those phases in 3V LiMn2O4. The cyclic deflectograms and cyclic voltammograms were obtained simultaneously. The potential ranges responsible for the Jahn–Teller distortion in 4V range, which takes place at the electrode surface, was determined by the charge versus. differential strain (dε/dQ) curve. The onset of the Jahn–Teller distortion was observed at the end of the cathodic scan, and the relaxation of the Jahn–Teller distortion was observed at the beginning of anodic scan. Furthermore, the onset of the Jahn–Teller distortion was found to be dependent on the lithium ion insertion rate, which was controlled by the scan rate.