Supercapacitor systems are gaining more and more usage in transport, renewable energy and pulsed power applications because of their high-power density and long cycle life. One major issue with uncharged supercapacitors directly connected to a high-voltage D.C. Supply is the high inrush current and increased component stress. This paper presents a literature review of pre-charging control strategy for a four-switch buck-boost (FSBB) converter supplying a high-voltage supercapacitor stack. Unlike conventional design approaches that focus on steady-state ripple constraints and hardware-oriented component sizing, the proposed framework explicitly exploits time-scale separation between switching dynamics, inductor current evolution, and supercapacitor energy storage. A reduced-order energy-based model is derived to show that the pre-charging process can be governed by a quasi-linear voltage evolution independent of switching ripple. Using this knowledge, a state-machine based control algorithm is presented, which is able to transition seamlessly between the buck and buck-boost states and keeps the charging current constant over the entire range of the output voltage. Implementation details are given to generate the PWM and duty saturation limits, while simulations confirmed that a continuous and linear constant current charging with smooth supercapacitor voltage increasing is achieved, with a continuous transition. A simple and physical intuitive algorithm for high voltage supercapacitor pre-charging.