I'd venture to say the demeanor of an INTJ 4 would actually look more INFPish (a REAL INFP, not the stupid stereotypes) than INFJish.
I'm thinking Trent Reznor is a good example of this (okay, he's probably a 5w4, but he's a Fi heavy INTJ, so much so he could easily be mistyped as INFP, IMO). One reason for this is Fi looks much colder than Fe.
My cousin is INTJ 5w4 (so not a 4 either....but close enough to make a point), and demeanor-wise, he's more like me than his INFJ mom (although she is 3w4 & not 4 either), but thinking-wise, he's probably more similar to his mom (they have a more "strategic" attitude towards life).
I'd still suggest to an INTJ who suspects they are a 4 to check the wings (5w4 & 3w4) & see if maybe one is not a better fit. I don't think reconciling INTJ + e4 is as hard as some other combos people claim (I've seen ESTJ e4 - REALLY? So obviously a 1 disintegrating into 4...).