The quickest method of differentiating ISTJs and INTJs is simply to ask if they like Monty Python skits. ISTJs detest that juvenile nonsense while INTJs will perk up immediately and start reciting lines.
Here is an interesting question, that needs to be answered... Do INFP 3w4s exist?
I would expect so. I find that Enneagram and MBTI aren't as correlated as most people think. While each is a personality typology, the personality descriptions are not the essential part of each typology. MBTI (using Jung's functions) is about the typical thought patterns a type engages in, while Enneagram is about the typical coping mechanisms a type has. I see no reason to assert that a particular type would never have a particular coping mechanism. Rather, I strongly suspect that MBTI/Enneagram types that don't match up personality wise tend to be more hidden, with one side or the other being more prominent in one's personality.
An argument is two people sharing their ignorance.
A discussion is two people sharing their understanding, even when they disagree.