Sounds to me CA remains the spouse's domicile and the spouse will, therefore, continue to be liable for CA tax during the 4 years of residency.

The spouse could possibly qualify for the safe harbor to be considered nonresident (so long as the days of presence in CA are carefully manage and the amount of intangible income doesn't cross the threshold) but there'll be another hurdle to overcome, which is over filing status because CA generally follows that of the federal return.

This will be a problem if your client has CA-source income, such as wages, that would be allocable to the spouse since the exception for using a different filing status mandates, inter alia, that the nonresident spouse must not have any CA-source income during the tax year.