The chapters in the original Volume III are: Theodosius, the destruction of Paganism, the sons of Theodosius, plus Rufinus and Stilicho, the Sack of Rome, Arcadius, St John Chrysostom, and Theodosius II plus Eudoxia and the eunuchs, the Vandals in Africa, Attila the Hun, Attila the Hun again, the end of the Western Empire, monasticism and Arianism, what was happening meantime in France, Spain and Britain, and finally some general observations on the fall of the Roman Empire in the West. I actually found it rather tough going in places, with a lot of background info for the main event, the Sack of Rome, and then the rather dull dying spasms of the western half of the empire. The concluding General Observations are rather good, and were written befor most of the rest of the book, so can comfortably be read on their own if you just want to see if Gibbon appeals to you.
The chapters in the original Volume IV are: Theodoric and Boethius, an introduction to Justinian, Belisarius' military successes in the west, Justinian's campaigns in the east, chaos in the west and natural disasters, Justinian as a lawyer, the Lombards and Italy after Justinian's death, the Persians, the Avars and the Emperor Heraclius, and Christianity in the East. I found it a much more coherent book, most of the chapters unified by the ambiguously fascinating characters of Justinian and Belisarius, and topped and tailed with Boethius and the Eastern Christians.
Anyway, 24 more chapters to go; I should finish it some time this year.