Sun in the Fourth House
Composite Sun in the fourth house favors a strong emotional involvement.
The fourth is the house of your most personal and intimate life, your home, your past and origins in general, as well as your inner feelings and emotions. Thus this position favors a relationship in which you settle down together and share each other's personal lives as intimately as possible.
This is one of the best positions for marriage. It indicates a concern for owning a home or real estate together, which may manifest itself as a desire to own land.
On the psychological level, you are likely to experience profound emotions together and to become deeply involved in each other's innermost mental and emotional depths.
Sun Conjunct Mercury
The conjunction of composite Sun and Mercury signifies that there will be a great deal of mental activity in this relationship. This aspect is good for verbal communication between you.
You both should be wary of the tendency to intellectualize your emotions rather than deal with them at the gut level.
You have an unusual similarity of thinking, or at least a better than average ability to understand each other's thinking.
Mercury is a planet of travel, not always in the literal sense of the word, but in the metaphorical sense; that is, you like to be continually exposed to new ideas and experiences together.
One of the most positive attributes of this aspect is the ability to be detached from what you are discussing. Because of this detachment you may even be able to talk about your relationship and arrive at understandings that will help you both to get more out of it.