"Are ISTPs actually closer with INTJ, ENTP and ENTJ than with INTP?"
I've found it pretty easy to share personal info with the first 3 types for sure, especially ENTP friends. I'm married to an ENTJ (he typed that way, I'm not sure he's the stereotype ENTJ tho).
The problem with ENTP friends is that with one of them, his loyalty was not with me. So I stopped talking to him. I couldn't tell him anything without him blabbing to other people. The other ENTP was fine tho. Good dude, no drama.
The INTJ was pretty easy to talk to as well. I feel like he kept everything confidential. The problem arose when his insecure gf had issues with us being friends. Being a horny dude, he ultimately chose to obey his gf and ditch me as a friend. Not officially, but that's pretty much what happened.
And ENTJ husband... Obviously I'm comfortable telling him things. He established very early on that his loyalty was to me. I definitely appreciated that after being burned by the crappy ENTP I described and the INTJ for ditching me like that when I have an ESTP who had many an argument with his gf about how she couldn't pick his friends.
If some of my friends in college are/were INTPs (I believe they tested that way back then)...then I'll say I could talk to them about a lot, but not everything. They were awkward about romantic stuff. Probably because at some point they had a crush on me. :rolli:
I have a very good friend who said she used to test as INTP when she was younger but tests as INTJ now...and she's freakin' awesome. I can talk to her about anything and she's a great friend.
I seem to like ETSx, xNTJ and xNFP types best.