With infrared spectroscopy, we studied the bulk electronic properties of the topological antiferromagnet MnBi2Te4 with T-N similar or equal to 25 K. With the support of band-structure calculations, we assign the intra- and interband excitations and determine the band gap of E-g approximate to 0.17 eV. We also obtain evidence for two types of conduction bands with light and very heavy carriers. The multiband free-carrier response gives rise to an unusually strong increase of the combined plasma frequency, omega(pl), below 300 K. The band reconstruction below T-N yields an additional increase of omega(pl) and a splitting of the transition between the two conduction bands by about 54 meV. Our study thus reveals a complex and strongly temperature-dependent multiband low-energy response that has important implications for the study of the surface states and device applications.