  In order to isolate the contribution of the nucleon strange electric form
factor to the parity-violating asymmetry measured in 4He(\vec e,e')4He
experiments, it is crucial to have a reliable estimate of the magnitude of
isospin-symmetry-breaking (ISB) corrections in both the nucleon and 4He. We
examine this issue in the present letter. Isospin admixtures in the nucleon are
determined in chiral perturbation theory, while those in 4He are derived from
nuclear interactions, including explicit ISB terms. A careful analysis of the
model dependence in the resulting predictions for the nucleon and nuclear ISB
contributions to the asymmetry is carried out. We conclude that, at the low
momentum transfers of interest in recent measurements reported by the HAPPEX
collaboration at Jefferson Lab, these contributions are of comparable magnitude
to those associated with strangeness components in the nucleon electric form
factor.
