Considering multi-domain TN displays, a four-domain structure gives the most symmetric viewing angle characteristics and gray scale performance. However, domain instability of the four configurations in the operating state is an obstacle for their practical application. In this paper, the origins of the instability in different four domain configurations are investigated. We evaluate the critical conditions for the domain stability using a simple model. The consequences of the model are tested by experiments, and methods to prevent the domain instability are suggested.