The specific reference here in Book IV of Paradise Lost is to Revelations 12:3 and St. John's vision of a new battle in Heaven. Milton opens the book with:
O for that warning voice, which he who saw / The Apocalypse heard cry in heaven aloud, / Then when the dragon, put to second rout, / Came furious down to be revenged on men... (IV.1-4)
In this book, Satan is newly arrived in paradise and Milton's narrator almost pleads for a "warning" for Adam and Eve. But he makes a reference to a much later happening when it comes to a biblical timeline, where Satan, in the form of a dragon contends in a "second rout", a second war:
And there appeared another wonder in heaven; and behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads. (Revelations 12:3)
But this time in Revelations, it is about parading in the guise of the Christ. However, this isn't a far stretch, since Satan will take on many guises throughout his stay in Paradise as he plots against God and manipulating Adam and Eve.