The other possibility is that "Aram" was a printer's error, and that Saadia is in truth referring to Edom, who is identified in Amoraic writings as Rome. The difference between the words Aram and Edom is the change from a "Reish" to a "Dalet," which look very similar.
(The identification of Rome with Edom itself is somewhat questionable, as Edom, the descendants of Esau, originally settled to the south and East of Israel, while Rome is far to the north and west of Israel. Ramban explains that since members of Edom were the first to embrace Christianity, the whole Roman Empire, which later accepted Christianity, is therefore christened (pun intended) Edom. Abarbanel surmises that Tzefo, a grandson of Esau, may have originally settled Rome, thereby causing its being named after Edom. He also says that it is possible that Edom is a foil to Judaism. It originally represented Esau, hater of Israel, continued with Rome, who took over Israel, and was also Christianity, which was perceived as a threat to Israel. Ibn Ezra takes a different approach. He conjectures that the term Galut Edom, exile of Edom, is actually a misnomer. Since the people of Edom were the first to celebrate the destruction of Israel, the entire exile was erroneously called in the name of Edom.).
Ibn Ezra identifies the third kingdom as the joint dominion of the Greeks and the Romans. This would then leave the fourth kingdom as Ishmael. Ibn Ezra explains that he could not say like Saadia that Greece alone is third and Rome and Ishmael are jointly fourth because the verse says that the first part of the split fourth kingdom will be stronger than the second. This seems incorrect because if Rome is first and Ishmael is second, Islam is larger than Rome (or at least ibn Ezra thought so). Another problem Ibn Ezra finds with Saadia is that it says that one group will be subsumed under the other, and the Roman and Moslem empires are not allies- in fact they were battling in the time that Ibn Ezra was writing.
