Transmission spectrum of WASP-4b from GMOS B600 data (blue triangles) and GMOS R150 data (red circles). The absolute levels of the B600 radii with respect to the R150 radii are plotted using the best-fitting ﹩{\rm{\Delta }}z﹩ parameter from the large-grain-dominated atmosphere fit. The uncertainties on the data points are the uncertainties on the wavelength-dependent ﹩{R}_{P}/{R}_{\star }﹩ only, and do not include the uncertainty on the absolute transit depth or the ﹩{\rm{\Delta }}z﹩ parameter. Also shown are the cloud-free forward atmospheric model from Fortney et al. (2010), an atmosphere dominated by scattering from small-particle grains and an atmosphere dominated by scattering from large-particle grains. Models are binned to the resolution of the data. Note that the lower precision redward of 700 nm is due to residual fringing and also because we were able to use only transit 3 for this wavelength range. Blueward of 700 nm, we combined the results of transits 1, 2, and 3.