Proper Titles for Cover Letters

This in’t exactly a technical question; however, I need assistance in determining the proper Title for a Cover Letter. The addressee has an MBA and is the Executive Director of a Research Foundation. The position is the Assistant to the Executive Director.

Should the letter be addressed to:

Dear Ms. First Last
Dear Doctor Last
Dear Director Last
Dear Ms. Last

Please help and/or direct me to the right search engine to ascertain this information. This i time sensitive – and your assistance is greatly appreciated.