So, lets see what Mercury can do in 4th house without any conjunction or aspect from any other planet. 1st lets see what these 2 things represent --
Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.
4th house represents your home, home environment, homeland, mother, nourishment, childhood friends, peace of mind, conveniences etc.
So, now the super communicator Mercury has come inside the home. 1st of all, it shows a highly communicative person in home environment. This person loves to talk to parents especially Mother. As Mercury also represents logical thinking and intellect, they imbibe their logical thinking and intellect from their mother. Their mother is very communicative too. As they loved communicating to their parents, same way they want to communicate with their children. Mercury is also a businessman and 4th house is home, so quite easily this placement of Mercury can make a person run business from home. But overall, communication within home environment become an integral part of the life.
Now, from 4th house, Mercury aspects the 10th house of Career. It can give few career options. 4th house is people of home land, 10th house is Government, Mercury is communication and speech, so political career. Mercury is Counselling, 4th house is home and 10th house is Career, so it can show someone who does Career Counselling for others.
Now, one career which will always come with almost all positions of Mercury is Astrologer. Because Mercury is the significator of Astrology. Mercury controls Astrology itself. And the reason is, Mercury represents Collecting Information, Logical Thinking, Intellect, Quick Decision Making, Counselling, Memory etc. and for being an Astrologer, you need exactly the same things. You need to 1st collect loads of information, then through your logical thinking filter useful information from junk, then use your memory to memorize charts, make a decision and counsel others. It becomes a more prominent career if Mercury is Amatya Karaka and somehow gets connected with Ketu (planet of mysticism) or 8th house or 8th sign i.e. Scorpio.
Hope this helps.
Tomorrow - Mercury in 5th house.
Vishal S Saxena - Astrologer