Ooops, late to the party. I've read the description that Southern Kross posted and it's really too negative on the 4w5s
Not as big a fan of the enneagram compared to MBTI and functions so what follows is possibly tainted from a mixture of sources
Why I feel I'm 4w5?
1) The individualism part
There is this need to be able to say that I did things my way. Not because I want to be different from everyone else. Ok, not just because of that, but rather because I prefer to make my decisions based mainly on my own criteria. Hence, I tend to be distrustful of things that are popular unless I have tried them myself. Take something subjective like movies or music for example. Just because it's a movie that is making record sales doesn't mean I have to like it. Of course, it doesn't mean that I automatically hate anything that is popular. Or that I have an elitist attitude toward things like commercial pop music. There are songs I like and songs I don't. Again, it's about making up my own mind about things, and not being contrary for the sake of being contrary.
2) The whole 'emotion' thingy
"I emote therefore I am" seems like the motto I live by. The mood I'm in is the main thing that helps me with my decisions. Instead of trying to put the emotions in the backburner and make a decision, if the emotions are not engaged then why bother? Yes, there may be times when I tend to over-wallow in emotions but I feel that it is an important side of me and ignoring the messages they are sending would hurt more than help. The day I stop being sensitive to my emotions will be probably the day I'm laid 6 feet under
3) The dark side
Like a Jedi, there is always a dark side to 4w5s

It makes for a lot of emo and sighing but to me a balanced life is one where you recognize both the good and the 'evil' within. The danger is when the shame from the perceived deficiencies over-ride every thing else and you feel inferior to everyone else. The trick is of course realizing that everyone else also their strengths and weaknesses, and envy and shame are just a waste of energy.