When Mercury is placed in the 4th house of a person's horoscope, it indicates that the person may be highly intellectual, curious, and possess strong analytical skills. The individual may have a keen interest in learning and acquiring knowledge, especially about their family and home life. 

The influence of Libra sign on Mercury in the 4th house indicates that the person may have a strong sense of balance, harmony, and diplomacy in their relationships with family members and those in the home environment. They may be skilled in resolving conflicts and mediating disputes amongst family members. 

This placement may also indicate that the person may have a deep connection with their mother, as Mercury rules over the mother in Vedic astrology. The individual may have unique or unusual ideas about home life and may enjoy discussing various home-related topics with their family members or roommates. They may also be interested in decorating their home or creating a comfortable and harmonious environment. 

However, if Mercury in the 4th house is afflicted, it may indicate communication issues within the family or a tendency towards manipulation or deceit within the home environment. The individual may also struggle with making decisions related to their home or family life.

Mercury rules 12th and 3rd house is in 4th house in Libra