This paper proposed a PV application based on a prototype of four-switch buck-boost (FSBB). Because this application usually faces the requirement that both step-up and step-down modes are needed in Maximum Power Point Tracking (MPPT) control for better efficiency and power density—especially in PV-UAV power systems where weight and volume are key parameters—in order to reduce inductor weight and improve efficiency at the maximum power point of the photovoltaic module, zero-voltage switching (ZVS) control schemes with high switching frequency are a good choice to reduce the loss of GaN devices. After reviewing many proposed control methods, a control strategy with quadrilateral inductor current is suitable for this application. Based on a simplified real-time control algorithm, an MPPT control scheme that reduces the RMS of the inductor current and realizes ZVS operation under 100 V–200 V input and output voltage is proposed, and the whole control is realized with simple calculations. Finally, a 400 W MPPT converter is built to test the proposed MPPT control scheme, and the controller is a low-cost DSP commonly used in power converters. The prototype achieves a power density of 4 W/g and a peak efficiency of 98.3%.