Four upper panels: individual lines of C₄S with J = N + 1 detected in our survey. Two bottom panels: spectrum obtained after stacking all lines of C₄S with J = N + 1 (panel b) and the resulting spectrum after stacking the twelve weak lines with J = N and J = N − 1 (panel c). The abscissa corresponds to the velocity with respect to the local standard of rest. The dashed blue line corresponds to v_LSR = 5.83 km s⁻¹. The ordinate is the antenna temperature in milliKelvin. The red line in the four upper panels corresponds to the synthetic spectrum for each transition computed with T_r = 7 K and N(C₄S) = 3.8 × 10¹⁰ cm⁻².