The specific energy loss of4He ions in silicon has been determined in the energy range 420–2750 keV. Several different methods of determining the specific energy loss were used to check the results. Studies were made of the variation of specific energy loss with silicon density and showed that lower‐density amorphous silicon has relatively higher specific energy losses. Studies were also made of the specific energy loss of4He ions in radiation‐damaged silicon, polycrystalline silicon, and single‐crystal silicon. The values were then used in a thick‐target calculation to generate spectra of4He backscattering from silicon targets. These calculations were compared to experimental spectra over a range of energies and backscattering angles. All calculations were within 2% of the experimental spectra. The ``random'' spectrum from a spinning crystal target was compared to thick‐target calculations and shown to be useful as a normalizing spectrum with an accuracy of ±5%.