He also was most certainly not thinking about any of the four Gospels which were not even written when he said this.
So, before you claim that the Bible is the Word of God because Paul claimed that "all scripture is God-breathed" be sure to remember that what he considered scripture and what you're currently referring to as scripture are not the same.
Also, remember that just because something is "God-breathed" doesn't mean it is infallible and inerrant. Humans are also "God-breathed" and we are neither infallible or inerrant.
Yes, scripture [whatever that might be], "isinspired by God and profitable for teaching, for reproof, for correction, for training in righteousness..." but determining how and in what ways scripture is profitable for us is something that must be discerned by the Holy Spirit of God that lives within each and every one of us.