If we assume that the Four Gospels were written independently*, they would serve as four independent witnesses.
This possibility can be put to rest rather easily. Just look at any synopsis of the synoptic gospels. A synopsis is a book in tabular format where the synoptic gospels are placed side-by-side. Passages relating the same thing are lined up next to each other, even if they don't appear in the same order within the gospels themselves. The large number of identical passages can be easily seen in a synopsis and renders any possibility of indendence moot.
Of course, the synoptic gospels do not include John, which has little in common with them beyond the main outline of the story.
Furthermore, the older the Gospel, the more sophisticated and complex is the narrative and information content.
Consistent with our understanding of how myths and legend build, I think the opposite is the case, i.e., Matthew and Luke both provide more detail than the earlier Mark, and they all provide more detail than the hypothetical 'Q'.
An aside to Thylacosmilus: The BoM's authenticity is problematic in the extreme. There were certainly never any golden tablets, making it highly likely that Moroni, and all that comes with him, is baloney. Consistently, though, the gospels are very likely also made up, meaning that the Jesus of the gospels never existed. Joseph Smith and Paul likely had much in common as mythmakers.