בלעם at the end of the parsha, discussed according to many, the Messianic era. The Ramban understands that he was talking about the ד' מלכיות. The Ramban quotes the Ibn Ezra who thought that ישמעאל was the last of the ד' מלכיות. The Ramban disagrees and says that the Ibn Ezra is completely wrong and that ישמעאל is independent of the ד' מלכיות.
I heard teh following explanantion of the Ramban. The ד' מלכיות represent rebellion against Hashem. They take the kingship from us (who represents Hashem in this world) and worship avoda zara. ישמעאל is different. ישמעאל has Hashem's name in his name and is not rebelling against the מלכות of Hashem. ישמעאל as the Torah says is a פרא אדם, a wild man. He believes in Hashem but wants to do what he wants to do. ישמעאל does everything in the name of Hashem including killing. Therefore, ישמעאל is independent of teh ד' מלכיות and can exist concurrently with them.