Option 4 is the correct answer. In the Manusmriti, the form of marriage that results from the "Voluntary union of a maiden and her lover" is the Sixth form. The Manusmriti is an ancient Hindu legal text that provides guidelines and rules for various aspects of life, including marriage. It describes eight different forms of marriage, each with its own set of conditions and rituals.

The Sixth form of marriage, also known as Gandharva Vivaha, is based on mutual love and attraction between the woman and the man. It is a form of marriage where the couple enters into a union without the involvement of their families or any elaborate rituals. It is considered a love marriage and is often associated with romantic relationships between individuals who choose to marry out of their own free will.

Therefore, option 4, Sixth form, is the correct answer, as it aligns with the description given in the Manusmriti.