4A molecular sieve is an alkali metal aluminosilicate with a pore size of 4 angstroms (0.4 nanometers), and it is a sodium-containing A-type molecular sieve. Due to its internal structure containing numerous 4-angstrom crystal adsorption cavities, it can adsorb molecules with a diameter smaller than 4 angstroms while not adsorbing any molecules with a diameter greater than 4 angstroms (including propane). It has a strong selective adsorption for water molecules, making it commonly used as a selective adsorbent and drying agent for various gases and liquids.