It seems to me, as a four: Sixes have this more heady energy- a lot more anxious and keyed up. It's like you can hear the gears in their head turning. Or there's this sort of nervousness. They seem more either cynical or skeptical (typically 6w5) or trusting and lighter/fun (6w7). They seem more human to me, if that makes sense. Sixes may rebel for the sake of rebelling- for example, shaving their head and getting tattoos just to 'freak people out'. There are a lot more authority issues, or issues with belonging, or issues with deliberately setting themselves apart. Sixes are more loyal and supportive/ desire loyalty and support than fours. They tend to think out loud or tell their thoughts to others in order to process them- the 6w7s, anyway.
Fours have more of a heavy heart energy- this regretful moroseness. When I think of four I think of the first verse of a poem by Paul Verlaine- Il pleure dans mon coeur comme il pleut sur la ville/ Quelle est cette langueur qui penetre mon coeur- It cries in my heart like it rains on the city/ What is this languor which penetrates my heart?
Fours tend to be more sullen or despondent. This isn't to say a four can't have fun and can't be silly, but there's always this undercurrent that the fun silliness is fleeting. Fours are separated from others, but they don't do this deliberately. They just are. They may shave their head and get tattoos but it's more about expressing something within. Fours don't rebel so much as just have a different way of seeing things/ doing things. Others may perceive this as rebellion, but it's not. Fours are self absorbed- by this mean, every thing comes back into the self- how does this affect me? How does this make me feel? How does this relate to my past and me feelings of regret, shame, envy? Fours can also be supportive, but often drop people when the relationship 'feels wrong'. They have higher expectations and become frustrated more easily.
More importantly though, you said you very much related to what I said about the four image- why the questioning?
My guesses:
ISFP Fours- Heath Ledger, Bob Dylan
ISFP Sixes- Kristen Stewart, Woody Allen, Julie Delpy