  We present an optical transmission spectrum of the atmosphere of WASP-4b
obtained through observations of four transits with Magellan/IMACS. Using a
Bayesian approach to atmospheric retrieval, we find no evidence for scattering
or absorption features in our transit spectrum. Our models include a component
to model the transit light source effect (spectral contamination from
unocculted spots on the stellar photosphere), which we show can have a marked
impact on the observed transmission spectrum for reasonable spot covering
fractions (< 5%); this is the first such analysis for WASP-4b. We are also able
to fit for the size and temperature contrast of spots observed during the
second and third transits, finding evidence for both small, cool and large,
warm spot-like features on the photosphere. Finally, we compare our results to
those published by Huitson et al. (2017) using Gemini/GMOS and May et al.
(2018) using IMACS, and find that our data are in agreement.
