Fourth house is the house of mother, motherland, heart, happiness and everything that makes a native blessed to be alive. In simpler terms it is the house of emotions and ruled by Moon. Mercury is analytical and associated with reasoning and logic. Emotions aren't something that can be measured by using analytical skills or by applying rational or by using logic. Emotions are pure and transcend human logic. And that is why Mercury feels uncomfortable in the fourth house. Not to mention Mercury thinks of Moon as his enemy. 

     Mercury in fourth house has been known to make the native pursue an educational field which is entirely different from his/her professional field. So if the native has studied engineering than he or she might have to work in a management role which is entirely an opposite area. Apart from this fourth house is the house of basic education. This basic and formal education shapes the thought process of the native for future. Mercury is a chatter box who may parrot off all the knowledge that a book has to offer but he does not understand the importance of it. That is why it is better off if Mercury is not in the 4th House and if it is placed then there should be Sun or Venus placed along with it to give it guidance. When Sun is placed along with Mercury than the native may become a very good administrator, diplomat and may even have success in agricultural field. 

     According to B. V. Raman those with Mercury in fourth house are: "Self-made man/woman, aren't afraid to criticise the authority, can work as a teacher, professor, lecturer or diplomats, have fondness for music and fine arts. They also frequently travel to other countries. They have a witty speech". Mercury is great as long as he is placed with Sun or Saturn in fourth house. Mercury with Moon isn't ideal as both are enemies and may give mental disorders in old age. With Venus, Mercury gives a career in arts, music or fashion or even as critic of them. Mercury with Jupiter in fourth house is great in friendly sign and makes one a smart and intelligent person but in a bad sign may make a con artist who makes a living by fooling others. Mercury and Saturn in fourth house gives a career in accounting, software development or engineering. Mercury along with Mars may make a doctor or hardware engineer. Mercury along with Rahu in fourth house gives mixed results. Mercury with Ketu in fourth house drives a person to be an obnoxios. Ketu is detachment and Mercury is articulate and analytical. Fourth house isn't the place where they are suited to be together. Natives who have Mercury along with Saturn, Mars, Rahu, Sun or Ketu in the fourth huse may have some troubles in old age.

     Mercury in fourth house is not the best placement for it. With Sun, Venus or Saturn it may provide some help. In general most natives do not pursue a career in which they have obtained education. But overlooking that scenario they still have wonderful careers.

Read our article: Remedies For Mercury to help with weak Mercury.

References:

Raman, Bangalore Venkata. How to Judge a Horoscope. Delhi: Motilal Banarsidass, 2014. Print.