Limited Proportional Region:

Within the limited proportional region (Region IV), as voltage increases, further processes occur leading to increased ionization. The tough field causes increased electron velocity, those results in excited states of higher energies capable of releasing more electrons from the cathode.  Those events cause the Townsend avalanche to spread along the anode. The positive ions remain near whereas they were originated and decrease the electric field to a point where additionally avalanches are impossible.  For this purpose, Region IV is known as the limited proportional region and that is not used for detector operation.