For me, I can possibly see an ISFp being a 4 if they have a strong Fe type. So if your friend is an ISFp, it's likely they are strong Fe type. You could check this out yourself by reading Filatova's sub type descriptions and seeing which one matches up.
I think that if one reads the 4 description, what we're really reading is almost like an INFp description..ie socionic blocked with And also that we are reading someone who is an IP with creative
You've put forward a good argument in general, and i've said something similar myself, I think I said previously that taking parts from an enneagram description is something like taking someones quote out of context and putting it in a newspaper or magazine.
The links are interesting, however I think that these correlations come from celebrity typings, which are harder to be sure of accuracy even than individual typings over the internet. So I think the best thing is to rely on reading both typologies and seeing yourself how it applies IRL with peoples types you can be more sure of.
Although I will say that i've focused more on some other correlations than a 4 and an ISFp/INFp, I think what i've said still makes sense. Perhaps it doesn't