Reversible hydration of carbon dioxide. Together with BCA1, involved in the CO(2) signaling pathway which controls gas-exchange between plants and the atmosphere by modulating stomatal development and movements. Promotes water use efficiency. H(+) + hydrogencarbonate = CO2 + H2O Interacts with DTX56. Strongly expressed in aerial tissues including leaves, stems, flowers and siliques. Accumulates in both guard cells and mesophyll cells. In plants lacking both BCA1 and BCA4, impaired CO(2)-regulation of stomatal movements associated with reduced beta carbonic anhydrase activity in guard cells, and increased stomatal density. Belongs to the beta-class carbonic anhydrase family. Truncated N-terminus.