In the four level laser system, the atoms from ground state E1 are raised to excited state E4 with the help of pumping. From the E4, the atoms decay to energy state E3 by spontaneous emission. The transition rate of atoms from E4 to E3 is much faster as compared to transition rate of atoms from E3 to E2. This is due to the reason that E4 is an excited state with life time of the atoms of the order of 10-8 seconds.
E3 is the metastable state. Thus the number of atoms in E3 exceeds the number of atoms in E2. The population inversion is achieved between E3 and E2. The laser action takes place between E3 and E2 by stimulated emission. The atoms from energy state E2 get deexcited to E1. The atoms from E1 are again pumped to E4.
Example: Helium-Neon laser is four level laser system.
Advantage of four level laser: