It seems that almost everything rests on assumptions... When reading, and indeed when originally making, the New Testament, our assumptions concerning relative authority, make a really Big difference to what we get from it.
Given that the Fourth Gospel is, by its own account, written by the disciple whom Jesus loved; it ought to have priority over all other parts of the New Testament. At the very least, and given it begins with the beginning of creation, it surely ought to be the First Gospel: first in position, first in composition, and first in authority due to its authorship.
However, if the Fourth Gospel had been placed first in position and authority, it would have framed the rest of the New Testament in ways that are very different from how Christianity evolved over the next many hundreds of years. As it is, the Gospels open with the three 'Synoptics' - Matthew, Mark and Luke - which are similar in structure and doctrine; that is, the accounts of Jesus open with the genealogy of Jesus leading back to the ancient prophets of the Old testament, and a version of the Nativity story.
Why are the Synoptics put first in sequence and in authority, when they do not even claim to be eye-witness accounts; and indeed have internal evidence of being compilations? - When by comparison the Fourth Gospel is a wonder of integration, harmony and unity!
(Except for Chapter 21, which seems to have been added some time after the death of Simon Peter; said to be in the early 60s AD.)
Unless we really disbelieve the claims of the Fourth Gospel - in which case it should not be in the Bible at all, since it is clearly dishonest - then it should be First.
Instead, the Synoptics are de facto given priority, by the simple means of claiming to regard all the Gospels as equal - or, indeed, especially among Confessional Protestants, inferior in authority to the Pauline Epistles.
Since the Fourth Gospel is qualitatively different from the Synoptics (and Paul's Epistles) in content, emphasis and several significant features; when it is regarded as 'equal' in authority, it is simply out-voted!
This means that, in actual practice (and for many hundreds of years), the Fourth Gospel (which ought-to-be First in priority) has-been and is merely fitted-into the other Gospels and/ or the Pauline Epistles; and any differences are explained-away.
This is simply a fact; the question is whether it is justified.
And that hinges on our understanding of what happened in the early 'post-apostolic' era of the Christian church - and to what extent it was divinely inspired, and to what extent it was human, flawed and corrupt.
Do we trust that the early and dominant theologians and church leaders were fundamentally correct? - I don't.
Do we trust that God inspired at least some translations of scripture to be sufficiently true? - I do: wrt the Septuagint, the Vulgate, Luther's and the 'King James'; which I regard as all equivalently valid (although not identical).
By these assumptions, we can trust and use scripture (in these four versions), overall; and we can (as I do here) use scripture as evidence against the compilers and interpreters of The Bible.
You may not believe my assumptions are correct - and I cannot argue for them with 'evidence', since they are assumptions - but this procedure is coherent and reasonable.