Charge-Dependent Effects in the Photodisintegration ofHe4

${}_{}{}^4{}_{}{}^{}\mathrm{He}(\gamma , p){}_{}{}^3{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\gamma , n){}_{}{}^3{}_{}{}^{}\mathrm{He}$ cross sections have been calculated for photon energies from the ($\gamma , n$) threshold to 34 MeV. Effects of Coulomb interactions, channel spin mixing, and mass differences have been taken into account using a coupled-channel continuum-shell-model calculation of $p$-${}_{}{}^3{}_{}{}^{}\mathrm{H}$ and $n$-${}_{}{}^3{}_{}{}^{}\mathrm{He}$ elastic and charge-exchange scattering. The ratio $\frac{\sigma (\gamma , p)}{\sigma (\gamma , n)}$ is predicted to be close to 1, except at energies very close to the ($\gamma , n$) threshold.