MnBi2Te4 has been established as the first intrinsic antiferromagnetic (AFM) topological insulator. Although the predicted exotic states such as quantum anomalous Hall insulator and axion insulator have been demonstrated in this material, the origin of the electronic bands is still unclear. Through an in-depth investigation by ARPES experiment and the numerical calculations, an upper bound of 3 meV for the gap size of the topological surface state (TSS) is estimated. Furthermore, we also reveal band chiralities for both the TSS and quasi-2D bands, which can be well reproduced in a band hybridization model based on the circular dichroism measurements. Importantly, we numerically simulate the energy-momentum dispersions based on a four-band model with an additional step potential near the surface. The step potential confines the spinorbit-coupled quantum well states in addition to the TSS, providing a possible microscopic explanation for the quasi-2D bands. In this study, we offer a solid step forward in reconciling the existing controversies in the electronic structure of MnBi2Te4 and provides an important framework to understand the electronic structures of other relevant topological materials MnBi2nTe3n+1