We measured the Fourier transform infrared (FT-IR) spectra of thermal oxides with various thicknesses, grown thermally on 4H silicon carbide (4H-SiC) substrates. For the thin (8 nm thick) thermal oxide, the transverse optical (TO) phonon peak frequency in the thermal oxide on the 4H-SiC substrate was observed at ~1080 cm-1 and was higher than that recorded in thermal oxides on a Si substrate (1074 cm-1). This shows that the thin thermal oxide was under compressive stress, calculated to be approximately 0.4 GPa, at the interface between the thermal oxide and 4H-SiC substrate. The shift of the TO phonon for s-polarized light was found to be larger than that for p-polarized light. In contrast, for the thick (85 and 130 nm thick) thermal oxides, the TO phonon peak frequency tended to shift toward lower frequencies with increasing oxide-layer thickness. By comparing the FT-IR and cathodoluminescence (CL) measurements, we conclude that the TO phonon redshift with increasing oxide-layer thickness can mainly be attributed to a corresponding increase in inhomogeneity in the thick thermal oxides.