What is book number four about?
Delving deeper, Book Four discusses the voluntary and optional prayers. The prophet used to perform them in such a way as to recommend them for Muslims to imitate him by learning and performing these optional and voluntary prayers. This way, Muslims get closer to Allah, gaining more reward and satisfaction from Allah. The book accurately assigns and describes these voluntary prayers in such an easy way as for Muslims to understand well. Throughout its meticulous thirty-one questions, Book Four explains the sunnan ar-Ratiba, the voluntary prayers the prophet was keen on. Also, the book states the time and the number of rakats of each voluntary prayer. In addition, the book describes the recitations and the supplications said and practiced in each voluntary prayer.
P.S. Book Number Five deals with:
Prayer as the “First obligatory Duty in Islam:”