The brothers, kayin and hevel, were already distanced and Hashem's encouraging words of caution and heartening attention to kayin was to no avail. The mounting tension is now described with intriguing brevity: (4:8) "Kayin said to Hevel, his brother. And it was as they were in the field, Kayin rose up against Hevel his brother and killed him."
What did Kayin say to Hevel? Why is the conversation worthy of record but its substance of little significance? There are many suggestions. Whereas Targum Yonasan details a philosophical debate about G-d, the afterlife, and providence, Rashi says that indeed there was no conversation of substance. According to Rashi, Kayin was merely setting the stage for the murder. A contrived conflict and heated confrontation would provide the pretext for what would follow.
In a not dissimilar approach, Ramban and Ohr Hachayim understand that the conversation was a strategy mean to draw Hevel into the field, have him relax his guard and make him vulnerable.
Ibn Ezra suggests that Kayin related Hashem's message to him. According to Tosafos Kayin sensed some joy in Hevel and that riled Kayin further.
Yet after all the suggestions are studied, the question remains: if the conversation was indeed noteworthy, as Targum Yonasan indicates, why is it not recorded? If the conversation was merely a strategy, then why mention what adds so little to the storyline?
It seems to me that the Torah is alluding to a sad but instructive truth. Two brothers are distanced. It may be that one has suffered a crushing and devastating disappointment and he sees his brother as having a role in that; it may be about finances; it might be about philosophy. Their arguments and confrontations may be very sad and the volume may become deafening, but their brotherhood is still promising because they are still talking.
It is only when they stop talking to one another, when there is no common language or when they simply cannot be bothered to find it...