What Is It About the Fourth Century?

Here's one I can't make head or tail of. It's from the concluding chapter of the memoir of Napoleon's (disastrous) Russian campaign, by the Philipe-Paul de Ségur where the narrator undertakes to wax philosophical over the meaning of big history:

After fifteen hundred years of victories, the Revolution of the fourth century (that of the kings and nobles against the people) had been overthrown by the Revolution of the nineteenth century (that of the people against the kings and nobles).

--Philipe-Paul de Ségur, Defeat: Napoleon's Russian Campaign 288
(NYRB ed. 2008)

WTF? I mean, I understand that "Revolution of the nineteenth century" stuff--certainly understand it in a abook about Napoleon. But what is this "revolution of the fourth century" stuff? I mean the only thing I can think of from the fourth century (in the Western world, at least) is the conversion of Constantine. But how to get from that to a "Revolution...of kings and nobles against the people" is quite beyond me. Any takers?