A speed-electromotive-force (EMF)-adaptive minimum-level voltage selector with an explicit voltage-margin deadband is presented for a four-phase 8/6 switched reluctance motor driven by a four-level shared-switch converter with a bidirectional buck–boost front end. The available phase-voltage states are \(V_{dc}+V_{cap}\), \(V_{dc}\), 0, and \(-V_{cap}\). An experimentally identified nonlinear flux-linkage map provides incremental-inductance and position-dependent speed-EMF information for online voltage-demand estimation. The controller applies the boosted level only when the estimated demand exceeds the main-source level by a prescribed margin, while \(-V_{cap}\) is prioritised after turn-off for rapid current extinction and energy recovery. Current-slope capability, front-end volt–second balance, auxiliary-link sizing, two-group current reconstruction, and device-voltage stresses are derived. Equal-condition benchmarking at 50-V main and 100-V auxiliary links gives maximum torque-ripple reductions of 6.5 percentage points over the speed sweep and 3.5 percentage points over the load sweep, with a 2.0-percentage-point improvement in the converter-oriented efficiency metric at rated speed relative to the AHB baseline. Reduced-voltage tests on a 0.9-kW prototype demonstrate multilevel current regulation, braking operation, balanced phase currents, same-group non-overlap, and measured AHB comparison.