KMT-2021-BLG-1391L is a Sun-like G-type star (about 0.37 solar masses, 5,778 K), 5,720 light-years from Earth. KMT-2021-BLG-1391L b is a 4.5-Earth-mass planet, at 2.290 AU from its star, discovered in 2022 via microlensing. Microlensing detections occur when a foreground star's gravity briefly magnifies a background source, revealing planets that are otherwise too faint and distant to observe directly. KMT-2021-BLG-1391L b circles in the outer system, comparable to the Jupiter region. Its low mass makes KMT-2021-BLG-1391L b a Neptune- or super-Earth-class world rather than a gas giant.