Fits of the SH⁻ stretching mode from laboratory NH₄SH:H₂O spectra (dark green solid trace) and the combination modes from laboratory CH₃OH spectra (dash-dotted red or pink trace) to the absorption complex between 3.8–4.2 µm observed toward four protostars. The fit SH⁻ features are isolated from the laboratory spectra via a local continuum subtraction and scaled with the same factors as those used to fit the 6.85 µm bands in Figure 9. The CH₃OH features are similarly scaled using literature CH₃ OH ice column densities or upper limits calculated from the 3.53 or 9.74 µm features. As the SH⁻ stretching mode and CH₃OH combination modes are some of the weakest spectral features of the investigated ices, both the NH₄SH and CH₃OH laboratory spectra are smoothed with a Savgol filter prior to fitting to remove experimental noise. Excess absorptions that is not fit with SH⁻ and CH₃OH features are fit with Gaussians (dotted orange trace) that could be attributed to a combination of H₂S and thiol ices. The full fit is indicated with a dashed light green trace, and the horizontal shaded red bars indicate 3σ levels calculated from RMS errors in the 3.72–3.77 µm range. Data from 4.03–4.18 µm (G395H gap in JWST data, low S/N region in ISO data) is excluded from the plot. Plots of the local continua applied to the observed spectra can be found in Appendix E.1.