Premise: The Chronicles of Thomas Covenant is a series of ten high fantasy novels written by American author Stephen R. Donaldson. The series began as a trilogy, entitled "The Chronicles of Thomas Covenant, the Unbeliever". This was followed by another trilogy, "The Second Chronicles of Thomas Covenant", and finally a tetralogy, "The Last Chronicles of Thomas Covenant".
Hypothesis: Donaldson wrote a tetralogy because he didn't want three trilogies