[en] In the present paper, we show that the Renner-Teller effect in tetra-atomic molecules is always accompanied by (avoided) crossings of potential surfaces corresponding to the electronic states that correlate with a spatially degenerate species of linear molecule. We restrict ourselves to Π electronic states and the lowest-order (harmonic) approximation. It is demonstrated that the simple model we employ, implicitly involves a diabatic transformation of the electronic states, which makes possible to remove the difficulties caused by the breakdown of the Born-Oppenheimer approximation at nonlinear as well as linear nuclear arrangements. The reliability of the model is illustrated on example of the X²Π electronic state of the HCCS radical. Numerical computations are carried out employing the data obtained in extensive ab initio calculations