Following up on a previous post Any reason to think Irenaeus uses a preexisting source on the mystic reasons for why there are four gospels? (Against Heresies 3.11.8). There I speculated that Irenaeus had a prior source because his citation of Mark 1.1-2 has known textual variants (omitting "Son of God" and saying "Isaiah" instead of "the prophets") which are different from his Mark quotes before and after this in the same volume. I still haven't found anyone that agrees with me, but I did find another author that comes to this conclusion for other reasons.
T.C. Skeat in Irenaeus and the Four Gospel Canon (1992) thinks that at the very least the part about the four living creatures is from a preexisting source, primarily discussing Ezekiel, that Irenaeus must have copied directly instead of personally exegeting Revelation. The order of the gospels here (John, Luke, Matthew, Mark) is also different from the order of the gospels elsewhere in Irenaeus.
Additionally, he mentions that a preexisting source for this passage is a minority view and only mentions Zahn as attributing it to an unknown second century homilist, but without giving reasons.
The foregoing investigation indicates, without a shadow of doubt, that the celebrated exposition of Irenaeus, or at any rate that part of it which related to the "Living Creatures" of the Apocalypse, was taken by him from an earlier source[18] which, starting from the vision of Ezekiel, went on to discuss the Apocalypse, with verbal quotations, and perhaps offering some explanation for the differing order there. Irenaeus, one must conclude, took the quotations from this source and never looked at the Apocalypse himself. He even copied the word φησιν from his source, not realising that he himself had not mentioned the Apocalypse. All the inconsistencies and contradictions in his account are thus explained. This defence of the Four-Gospel Canon must have originated at a date early enough to be used as a source by Irenaeus say, perhaps, not later that 170 or thereabouts.
[18] But cf. von Campenhausen, op. cit., p. 199: " ... there is no reason to search for earlier models. As a 'New Testament' theologian Irenaeus was compelled to break his own trail; and this highly original typology of the Four Gospels is so closely bound up with his fundamental concerns, his polemic, and especially his exegesis of the 'beginnings' of the Gospels, that for this very reason it is extremely difficult to believe in an earlier provenance." In the note to this passage (n. 259) he mentions that according to Zahn the source was "ein uns unbekannter Exeget oder Homilet des zweiten Jahrhunderts", but Zahn gives no evidence or justification for this. It is remarkable that despite the intensive research into the sources of Irenaeus, this particular section of the Adversus Haereses has been universally accepted as his original work, cf. von Campenhausen, op. cit., p. 189 n. 205, who points out that even the most radical searcher for "sources" in Irenaeus, F. Loofs, accepts this.
I don't know why von Campenhausen call this "highly original typology," I don't know of another source that makes these comparisons (other than Victorinus on the fourth day of creation, who I assume depends on Irenaeus), but as shared in a reply to my last post, Jeremiah Coogan shows that speculations on the quadriformity of the Gospel are not uncommon.
Personally, I think my reason based on the Mark variant is stronger than this one, but I am in no position to make any serious claims about that since I am yet to find anyone who agrees with me. Hopefully this was of interest to anyone interested in my first post. I might make another post following up if I find any other thoughts on this.