Source: Michael E. Stone, "Apocalyptic Literature," Jewish Writings of the Second Temple Period, ed. Michael E. Stone (Philadelphia: Fortress, 1984) 428-429.
"Classic in this connection is 4 Ezra 14 claiming Mosaic authority, indeed authority beyond that of Moses, for the apocalyptic revelations...It has also been claimed persuasively that the methods of exegesis in the apocryphal literature in general and a fortiori in the apocalypses, show that the possibility of inspiration and the results of independent individual cogitation were accorded more weight than in rabbinic literature, this also leading to a less intimate tie to the biblical text."
Much of IV Ezra was written in the first century C.E., since it mentions Roman emperors from that time. Although there was a biblical canon--as Philo, Josephus, and the New Testament make clear--there were still Jews who believed that God continued to inspire new writings. I guess that's not too surprising, since the New Testament made the same claim!
This overlaps with my Fishbane paper. (BTW, I haven't written any Fishbane posts as of late because I've actually been working on the Fishbane paper.) During the post-exilic period, there were people who challenged the laws of the Pentateuch. They claimed new inspiration, and they acted as if their writings superseded what came before. That seems to be what occurred within certain circles in the first century C.E., as we see in writings such as IV Ezra.
At the time of IV Ezra, there was a clear canon that most Jews deemed to be authoritative, and it included the Torah and the prophets. Yet, there were still Jews who claimed new revelation, notwithstanding the canon's existence and prominence. Consequently, even though there were challenges to the Torah in the post-exilic period, it may very well have been an authoritative and widespread traditum, meaning that Fishbane's model of a traditio interpreting an authoritative traditum has merit. But Fishbane should still acknowledge that there were Jews who questioned the Torah, believing that God could act in new, fresh ways anytime he chose.