The viscoelastic moduli (elasticity and dilational viscosity) of monolayers of P4HS have been studied over a broad frequency range (0.1 mHz-200 kHz) using a combination of relaxation and capillary waves techniques. The analysis of the surface pressure, the elasticity and the viscosity in the semidilute regime show that the air-water interface is a poor (near-Theta) solvent for the monolayer. The results of viscoelastic moduli show that there is a broad relaxation process in the low-frequency range (omega <1 Hz), and another very intense relaxation process centered in the kHz region. This behavior contrasts with the one previously found for PVAc, a polymer for which the interface is a good solvent. For PVAc the relaxation found at low frequencies is much narrower, and two processes are clearly distinguished at higher frequencies: one centered at 500 Hz and another one at 40 kHz.