In listening to the debate between Sam Shamoun and Shabir Ally (link to debate), I was struck by an odd position implied by Shabir Ally's argument.
Shabir Ally argues that Mark is the first gospel and the other three gospels show evidence of a progressive trend, with John having the "highest" Christology. Indeed, he sometimes even accuses the other synoptic gospels of omitting words or changing words that are found in Mark so as to move toward divinizing Jesus.
On the other hand, forced by Sam Shamoun, he concedes that Mark is not a "Muslim Gospel." So what is Shabir Ally's theory regarding what happened to the Muslim Injeel ("gospel")? That seems to be a tricky problem for Shabir Ally. Why on Earth would the Word of God be completely lost while various revisions of a false gospel be maintained?
Also, why would the earliest Christians have attempted to preserve all four gospels, if they were simply revisions of one another - or if the Christology of Mark were too low? To put it another way, if Matthew were really just an editing of Mark, why wouldn't Mark just be thrown away or suppressed?
There is really not a consistent theory of the textual transmission that makes sense from the Muslim standpoint. Basically, the Christians have to become experts at eliminating the true Word of God (such that it goes out of mention immediately, and none of the proto-Muslims are able to preserve even one copy of the Injeel), but for some reasons the Christians don't eliminate Matthew, Mark, and Luke (or any of them) but maintain them.
Furthermore, the earliest Christians don't even try to hide them - we find references to the four-fold gospel quite early in the patristic literature: Irenaeus died 202 and referred to the fact that the gospels are four in number, and argues that they cannot be more or fewer than four. (link)