The schematic energy level diagram of a four level laser is shown in figure 2.7.

Compared to the equivalent diagram of a three level laser, there is an extra energy level above the ground state. This extra energy level has a very short lifetime.

 The pumping operation of a four level laser is similar to the pumping of a three level laser. This is done by a rapid population of the upper laser level (E₃), through the higher energy level (E₄).

The advantage of the four level laser is the low population of the lower laser energy level (E₂).
To create population inversion, there is no need to pump more than 50% of the atoms to the upper laser level.
The population of the lower laser level (N₂(t)) is decaying rapidly to the ground state, so practically it is empty. Thus, a continuous operation of the four level laser is possible even if 99% of the atoms remain in the ground state (!)