Mass spectrometry (MS)-based protein footprinting, a valuable structural tool in mapping protein-ligand interaction, has been extensively applied to protein-protein complexes, showing success in mapping large interfaces. Here, we utilized an integrated footprinting strategy incorporating both hydrogen-deuterium exchange (HDX) and hydroxyl radical footprinting (i.e., fast photochemical oxidation of proteins (FPOP)) for molecular-level characterization of the interaction of human bromodomain-containing protein 4 (BRD4) with a hydrophobic benzodiazepine inhibitor. HDX does not provide strong evidence for the location of the binding interface, possibly because the shielding of solvent by the small molecule is not large. Instead, HDX suggests that BRD4 appears to be stabilized by showing a modest decrease in dynamics caused by binding. In contrast, FPOP points to a critical binding region in the hydrophobic cavity, also identified by crystallography, and, therefore, exhibits higher sensitivity than HDX in mapping the interaction of BRD4 with compound 1. In the absence or under low concentrations of the radical scavenger, FPOP modifications on Met residues show significant differences that reflect the minor change in protein conformation. This problem can be avoided by using a sufficient amount of proper scavenger, as suggested by the FPOP kinetics directed by a dosimeter of the hydroxyl radical.