Not a four, but I don't agree fours are boring when unhealthy. Boring is the territory of the "every man" 6 :P (I am a six, so I can say that... for the most part I wouldn't care/don't think sixes would mind being referred to in this way, whereas fours would). I do agree that sixes are very frustrating to deal with when unhealthy, moreso than fours.
What is the diff between NF 6 and the 4? I don't think that post really gets to the point on answering the question. It is comparative but not necessarily in the most helpful way.
I think the core differences between an NF 6 and 4 would still be the core differences between 4 and 6. I am very close to a four who is the same mbti type as me (infp) and we are still very different. It has little to do with the ability to affect other peoples emotions, etc. It's more to do with our internal life... how we deal with emotions differently (I haven't seen any difference in expansiveness of emotions, I would say that he is not self absorbed any more than me, nor would I say he lives in a less rich world of emotion). He ruminates, whilst my consciousness is split between future and a void. He has a much closer relationship with films, music, memories (they are -him- and how he understands the world) whilst I explore that to explore my emotions and "feel alive, and myself".
He is much more adverse with contending with the real world, whereas I am much more comfortable dealing with things as they come. He is gentle, with a caustic side that is often hidden, whereas I feel less gentle, but I have less bite. Being around fours feels soothing, being around me is more... there's just a different quality. Infps' are healers... i'd say he is the true healer, whereas i'm more likely to identify with "hero". There's something about fours that makes you want to look after them.... protect them, even if they mess up. They have a deep well of loving, that is diff. from the way an NF six loves imo. I think fours can represent both the best of both masculine and feminine love... whereas, my heart is quite distinctly masculine, even if on the outside my infp-ness is quite soft. I am a little more robotic than he... more concentrated on responsibilities, what people expect or want from me... more attached than I realise. He is less attached than he thinks.
He is true idealist, true infp, true seeker, I am more of a bastardised version of the nf in the sense that I am like a hybrid of intuiting, sensing, thinking, feeling, percieving, judging... he has a very true personality that feels untainted by the world. I am the product of having to pick up various abilities in order to contend with the world... I am not pure infp; I just couldn't afford to not fill in my own gaps in thinking and judging. I am the TJ version of infp, (the infp most infps would consider a sell out). I aim to be everything I am not, subsume my opposites; become them, in order to survive. He loves yet hates his opposities, a kind of hate and envy that remind him of his own personal gaps. I don't really feel envious of others... i'm more likely to put them on a pedastal to remind me of where I need to head in my personal life; what I already have inside of me, if I only looked. I believe what a person sees in others is what they have inside of them, whether they are aware or not.