Four-body Gravitational Microlensing Events Involving Both a Binary Lens and a Binary Source

We present detailed analyses of three anomalous microlensing events—KMT-2021-BLG-0209, KMT-2021-BLG-0901, and OGLE-2025-BLG-0356—identified from a systematic reexamination of KMTNet light curves for which previous modeling attempts failed or left persistent residuals. Although all three events show caustic-related features consistent with binary-lens microlensing, we find that their full light-curve structures can be described by four-body configurations that required four-body configurations involving a binary lens and a binary source (2L2S). In KMT-2021-BLG-0209, weak caustic-exit residuals arise from a faint companion source undergoing an additional caustic interaction. In KMT-2021-BLG-0901, a late-time rebrightening is produced when the secondary source encounters the resonant caustic long after the primary. For OGLE-2025-BLG-0356, we test the degeneracy between system is described by a triple-lens single-source (3L1S) and 2L2S interpretations of a short isolated anomaly and find that the 2L2S model provides a significantly better fit. Source colors and magnitudes indicate binary sources composed of (G8V, M3V), (G8V, K2V), and (G6V, G8V) stars for the three events, respectively. Bayesian inference suggests that the lenses are predominantly low-mass binaries, including one system (KMT-2021-BLG-0901) with a companion consistent with a brown dwarf. These events add to the growing sample of well-characterized 2L2S systems and underscore the importance of systematically testing complex models, particularly in anticipation of the high-precision microlensing data expected from the Roman Space Telescope survey.