This paper presents a bidirectional 4-level flying capacitor multi-level (FCML) converter for high voltage (> 1 kV) step-down/step-up conversion needed in hybrid rail and electric vehicle applications. Previous work achieved similar input voltage requirements through increasing the number of levels in the FCML, at the cost of additional transistors, flying capacitors and control complexity. In this work, we investigate the application of 650 V GaN transistors and apply a cascaded bootstrap method to supply the isolated gate drivers. We also analyze the performance benefits and limitations of zero-voltage switching (ZVS) in FCML converters. The design employs a single-sided PCB to improve the design flexibility for backside heat sinking. We demonstrate the design with an experimental prototype that achieves up to 1 kV operation at 2 kW with excellent flying capacitor balancing, a peak efficiency of 98.4% and a power density of 1500 W/in3.