I am setting forth pertinent provisions of 1203.4. If you read them carefully it requires you to disclose the conviction if you apply for public office, state licensure or contracting with state lottery. It does not require disclosure for private employment. I would not disclose the conviction on a job application.
the court shall thereupon dismiss the accusations or information against the defendant and except as noted below, he or she shall thereafter be released from all penalties and disabilities resulting from the offense of which he or she has been convicted, except as provided in Section 13555 of the Vehicle Code. The probationer shall be informed, in his or her probation papers, of this right and privilege and his or her right, if any, to petition for a certificate of rehabilitation and pardon. The probationer may make the application and change of plea in person or by attorney, or by the probation officer authorized in writing. However, in any subsequent prosecution of the defendant for any other offense, the prior conviction may be pleaded and proved and shall have the same effect as if probation had not been granted or the accusation or information dismissed. The order shall state, and the probationer shall be informed, that the order does not relieve him or her of the obligation to disclose the conviction in response to any direct question contained in any questionnaire or application for public office, for licensure by any state or local agency, or for contracting with the California State Lottery.
The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change.
I would need a few additional facts to be confident of my answer. If you were put on probation and imposition of sentence was suspended and you completed probation after which the reduction occurred PC § 17(b)(3) provides as follows: "When the court grants probation to a defendant without imposition of sentence and at the time of granting probation, or on application of the defendant or probation officer thereafter, the court declares the offense to be a misdemeanor..." the conviction is a misdemeanor for all purposes. In that case you could answer no to the first question and to the second "yes I was convicted of a misdemeanor but the conviction was dismissed." Some attorneys think that 1203.4 relief entitles you to answer no to the second question as well but the CA Judicial Council says you must answer as I indicated.
I want to amend and supplement my answer. To be precise a private employer is not allowed to demand information with regard to convictions where 1203.4 relief was granted. Caution is key here, however, as many large companies use private investigative services for job applications. Since your criminal record still exists and is not sealed by 1203.4 I believe the safer practice is to disclose the conviction indicating that it has been dismissed. Then no one can accuse you of lying on your application.