Concerted depolarization and Ca2+ rise during neuronal action potentials activate large-conductance Ca2+- and voltage-dependent K+ (BK) channels, whose robust K+ currents increase the rate of action potential repolarization. Gain-of-function BK channels in mouse knockout of the inhibitory β4 subunit and in a human mutation (αD434G) have been linked to epilepsy. Here, we investigate mechanisms underlying the gain-of-function effects of the equivalent mouse mutation (αD369G), its modulation by the β4 subunit, and potential consequences of the mutation on BK currents during action potentials. Kinetic analysis in the context of the Horrigan-Aldrich allosteric gating model revealed that changes in intrinsic and Ca2+-dependent gating largely account for the gain-of-function effects. D369G causes a greater than twofold increase in the closed-to-open equilibrium constant (6.6e−7→1.65e−6) and an approximate twofold decrease in Ca2+-dissociation constants (closed channel: 11.3→5.2 µM; open channel: 0.92→0.54 µM). The β4 subunit inhibits mutant channels through a slowing of activation kinetics. In physiological recording solutions, we established the Ca2+ dependence of current recruitment during action potential–shaped stimuli. D369G and β4 have opposing effects on BK current recruitment, where D369G reduces and β4 increases K1/2 (K1/2 μM: αWT 13.7, αD369G 6.3, αWT/β4 24.8, and αD369G/β4 15.0). Collectively, our results suggest that the D369G enhancement of intrinsic gating and Ca2+ binding underlies greater contributions of BK current in the sharpening of action potentials for both α and α/β4 channels.