This article presents a 0.4-V startup, dead-zone-free, monolithic four-mode synchronous Buck-Boost converter. By introducing constant frequency transition-Buck (T-Buck) mode and transition-Boost (T-Boost) mode in the transition region, the dead-zone problem of the dual-mode Buck-Boost converter and the inefficiency of the single-mode converter can be effectively solved. In addition, this article adopts the adaptive peak and valley current control and designs a reused slope compensation circuit, which reduces the converter’s static power consumption and improves the light load efficiency of the converter. To widen the input voltage range of the converter, a low-voltage startup scheme without off-chip auxiliary components and low- $V_{\mathrm {TH}}$ MOSFETs is proposed. The startup voltage is reduced to 0.4 V at a lower cost, and it greatly broadens the input voltage range of the monolithic converter. The proposed converter has been implemented in a standard 0.18- $\mu \text{m}$ CMOS process occupying a die area of 1.7 × 2.0 mm. The test results show that the converter with an input range of 0.4–3.3 V and an output voltage of 1.8 V. The peak efficiency can reach 90.8% at 100 mA and the light load efficiency can reach 85% at 10 mA. Besides, the efficiency in the transition region of 1.44–2.25 V is higher than 80%.