Alright so for starters a 4 is a very tough cookie, they're melancholic by nature but they are very hard to break because of how misery enriched they feel their existence is, they tend to be very emotionally open and very open to the emotions of others. The essential fear of the 4 stems from a fear of shame since that is the rudimentary fear for all of the heart types and for the 4 specifically the fear of shame molds into a fear of having no identity or personal significance, which manifests as a core desire to be special, original and different from others, as if to say "I'm flawed but I'm special."
The 6 is a much different breed, the 6 is a head type and therefore has a core fear derived from the fear of fear itself and of all the head types the 6 is the most fearful. The 6 basically sees the world as a scary place and feels that they are unable to trust themselves and so they go about finding others that they are able to trust. Essentially the life mantra for the 6 is team work, they fear being without support and see others as a kind of safety net. Now, there are two different kinds of 6, phobic and counter phobic, the phobic 6 gives into the fear more whereas the counter phobic 6 tries to battle against it. This could manifest in many ways but I look at it like this — I have a grandma who is a counter phobic and a best friend who is a phobic and the difference is that I almost typed my grandma as an 8 whereas my friend I almost might type as a 4 if I didn't know any better.
In essence, the reason a 6 might look like a 4 is because the 4 is kind of like a bleeding emotional wound and the six is kind of like a brittle mess, so they look similar because of the way that they both might seem a bit vulnerable, however the brittle nature of the 6 makes them fragile in a way that the 4 absolutely is not. They might both be prone to having breakdowns even, but the 4 is probably being melodramatic about emotions whereas the 6 is having a nervous breakdown. Controversy, a 6 might look like an 8 because if the six is counter phobic they will be rather bold and outgoing, my grandma at 60 some years old wrestled a 20 something year old down to the ground of a bar in Mexico on vacation over a free T-shirt and won, a phobic 6 is too busy being delicate and innocent and would be far less inclined to consider doing something like that.
What will help a lot with officially diagnosing your type is deciphering what your variant stacking is. A sexual 6 will be more counter phobic whereas a self preservation 6 will be more phobic, so a phobic 6 might look like a social 4 maybe whereas a sexual 6 might look like a self preservation 8 or something of that nature. The closeness of the 4 and the 6 probably becomes the most intertwined in regards to maybe the sexual 6 and the 4 for this reason. 4s like to rebel for the sake of rebellion and feeling alive whereas counter phobic 6 might rebel for the sake of trying to show how unafraid they are, like my grandma who is terrified of roller coasters for fear of being derailed going on one anyway just to prove she wasn't going to let her fear get the best of her, her behavior could easily be mistaken for that of a 4 or an 8.
It all comes down to identifying the key differences between them. The first is that 6s are quick and keen to form close alliances, hence why they are labeled the loyalist, whereas 4s feel that they are roughing it alone in life and would be prone to dress up in all black clothing and paint their nails black, earning themselves the title of the individualist. The second is that in general, 6s tend to be performing artists, while 4s tend to be original creators, so while both can have a creative streak in them, the 4s fear of being ordinary will lead them to be a different kind of creative. The third is that while both types can be inclined toward depression a 6 is more apt to get upset over anxiety or failure to please authority figures whereas the 4 will just become disenchanted with life itself more.
It is important to take into account that both types have a connection to 5 as well and that a 4w5 and 6w5 will have a lot in common with one another and tend to have a stronger connection to the depressive dark side of their types as those characteristics are likely to be magnified by the presence of a 5 wing, the 6 may even look introverted with this wing even if they're not. Even a 4w3 and 6w7 might look similar, 3s and 7s are often mistaken for one another because of their zeal and tendency to sparkle, although the sparkle of the 3 is much more contrived. As an INFJ 3w4 sexual/self preservation type for example I look a lot like a 4 but at first glance I also look like my best friend's type, ESTJ 6w7 self preservation/social. This is because sexual 3s deal with issues of gender identity and try to become a catch to override their fear of being worthless. Meanwhile the self preservation 6 seeks to look as innocent and sweet as possible to attract protectors. She and I therefore both look very feminine and both have a lot of sparkle and energy as well as a tendency to have minor mental breakdowns.
Despite this however, she and I are entirely different from one another, mostly because of our motives for being the way we are. I want to be outgoing because I want to attract attention, but I have a social blind spot and really could care less about having strong ties to any particular social group, I only care about bonding intensely with a few select individuals. Meanwhile my friend doesn't really care about attention but wants to have a place in many different social groups kind of like a safe haven, so even though she has a sexual blind spot she gives the appearance of caring an awful lot about one on one connections. I want to look feminine and girly to be sexy, she does it to look innocent and helpless, but since I want to mold to the idealized feminine stereotype innocent is sexy so once again we look the same even. The energy of my 3 vibe is easily matched by her 7 wing and we both seem very hyper even though I am a sexual type and she has a sexual blind spot.
In conclusion, from my experience 6s will look a lot more rational even when creative and will be a different type of creative than a 4 would be. Even if they both can have minor breakdowns, the 6 will be an anxious mess and the 4 an emotional mess. Your best bet at telling them apart is really just looking for the motives behind what they're doing.
Hope this helps!
