Because there are so few things that are clear cut in the enneagram, it's not safe to say "X sociotype cannot be X enneagram type". "Never" or "always" or "will" or "will not" does not really exist in the enneagram; "mostly" and "probably" do though. With a system like that in which most things are not clear cut I find it better to not make cross-system conclusions on what makes sense theoretically (because the theory is not clear cut enough to do that) but on the subject of the theory! The people! There are a lot of things that make sense theoretically but in the real world they don't necessarily ring true. The only way to ever know with absolute certainty that whatever sociotype could absolutely not be whatever enneagram type would be to study every person on the planet.
Also with the things that ARE clear cut...well lets take the assumption that an INFj cannot be an enneagram 4...the things that are clear cut when it comes to what it is to be a 4 are as follows:
An enneagram 4 must:
*Want/need/seek to discover or make an identity for his or her self
Actually I can't think of anything else that is an absolute must to be an enneagram four. (If there is feel free to correct me.) All the other 4 information seemed to fall under the category of "If most of this is true for you then you are probably a four." Which of course means that each bit of information is not a requirement in itself to be a four but only one point in the direction of being a 4 IF most of those bits are true. So the only ways in which an INFj could not ever possibly be a 4 would be if:
*Part of what is is to be an INFj is to NOT want, need, or seek to discover or make an indentity for oneself. (To my current knowledge the aforementioned statement is not in any INFj descriptions.)
*The clear cut requirements plus any combination of most of the things that are often true for enneagram 4s are in opposition of what it is to be an INFj. (I haven't found this to be the case.)
Because what is required for it to be impossible for an INFj to be a four does not seem to be in existence the only option for proving that would be, as I mentioned before, to research every INFj on the planet.
All of what I just said could also be applied to 4s and ISFps. In which I feel I have a much stronger case because I know of an ISFp that is an enneagram 4!! I talked to her and she said that she has been interested in the enneagram for about a year now and has read everything about it that she can get her hands on and strongly identifies with the enneagram 4. She said that learning about enneagram 4s was an eye opening experience for her because she had been under a few delusions that fours often find themselves under and that she applied some of the advice on enneagram self-help sites and it has been helping her. In terms of percentages, she said that with all the descriptions she's read when it comes to things true for her, the enneagram four description always had the highest percentage of things that are true for her in comparison to the other enneagram descriptions.
And actually I shouldn't have had to say most of the things in that paragraph because figuring out your E type is not rocket science!! It's usually not that hard!! If someone says they’ve done a lot of research and identify with a certain type then that alone is enough!
I'd also like to add that although most of you seem to prefer socionics over the MBTT system, the MBTT does stand on it's own. As a whole it functions and is valid. And in this system with 16 distinct different personalities there is a great deal of variation when it comes to what MBTT type can be what enneagram type. ( Click here to see: http://www.the16types.info/vbulletin...14&postcount=4 ) If that same variation were in socionics I wouldn't be surprised at all. I haven't read any in depth arguments about why SEI is incompatible with E4 but I imagine that the degree in which an ISFp seems like it would not be able to be an enneagram four, an (MBTT) INTJ would seem even less likely to be able to be a four but an INTJ can be a 4 because I've spoken with one and because of evidence here: http://www.geocities.com/lifexplore/typecorr.htm