INFP could be either 4ish or 9ish (though 4 in this case was the wing). The ISF/INP group (the “Behind the Scenes†interaction style) can be either the classic “Phlegmatic†temperament, or another one, called “Supineâ€, which is similar, but has more of an emotional energy than Phlegmatic. INTP tends to be more Phlegmatic, while INFP is often Supine. So even without the enneagram, a Supine INTP will seem more INFPish (and struggle with T/F).
9 is very Phlegmatic, while Supine would fit 6, along with 4 and 2. While the most stereotypical INTP will usually be strong 5, which itself is neither Supine nor Phlegmatic, but sort of mixes the Phlegmatic's reservedness with the criticalness of the NT (Choleric).
I would think an INTP 9 might be a less “seriousâ€, but still Phlegmatic (calm, cool) version of the type, while the 4 (or 6) would be more Supine. Now, with the 1 wing, that will add some of the seriousness back, but the motivations will be I imagine different from the standard 5.
5 with the 4 wing will likely be a softer, more reflective version, and inbetween Phlegmatic and Supine.
While the ISFJ is also likely more Supine, they are probably more on the 2 side, along with 6 and likely also 4. The Phmegmatic variant would be 9, but a 1 wing would also likely go with the J-ishness. So a 9w1 INTP would be similar to a 9w1 ISFJ, but not as much to other ISFJ's. I think the SJ (as a kind of Melancholic) can also match 5, so I would think the 5w4 INTP might be more like the ISFJ, where the 9w1 would be a bit closer to the stereotypical INTP. It might also be more liike the typical INFP, where the 9w1 would be more like the less typical Phlegmatic INFP.