  Despite the rapid progress in understanding the first intrinsic magnetic
topological insulator MnBi$_2$Te$_4$, its electronic structure remains a topic
under debates. Here we perform a thorough spectroscopic investigation into the
electronic structure of MnBi$_2$Te$_4$ via laser-based angle-resolved
photoemission spectroscopy. Through quantitative analysis, we estimate an upper
bound of 3 meV for the gap size of the topological surface state. Furthermore,
our circular dichroism measurements reveal band chiralities for both the
topological surface state and quasi-2D bands, which can be well reproduced in a
band hybridization model. A numerical simulation of energy-momentum dispersions
based on a four-band model with an additional step potential near the surface
provides a promising explanation for the origin of the quasi-2D bands. Our
study represents a solid step forward in reconciling the existing controversies
in the electronic structure of MnBi$_2$Te$_4$, and provides an important
framework to understand the electronic structures of other relevant topological
materials MnBi$_{2n}$Te$_{3n+1}$.
