Kind of a Robot
Of all the jurors, #4 is the nearest to a robot. The guy doesn't even sweat when all the other guys have made their shirts see-through. He's pretty convinced of a Guilty verdict in the beginning, but only because he believes in the power of evidence, while the others who cling to a Guilty verdict tend to have some personal bias against the defendant.
Whenever an argument breaks out, #4 is always quick to say stuff like, "I don't see any need for arguing like this. I think we ought to be able to behave like gentlemen." But when it comes to reaching a verdict in the case, #4 is completely unsympathetic, saying, "The boy's entire story is flimsy. He claimed he was at the movies. That's a little ridiculous, isn't it? He couldn't even remember what pictures he saw."
While he might seem cold and harsh, Juror #4 is actually not all that bad. For one thing, he's totally willing to be swayed by evidence. He tosses aside some of the early arguments about the defendant's innocence not because he's prejudiced, but because he doesn't believe them: "Everyone connected with the case identified this knife. Now are you trying to tell me that someone picked it up off the street and went to the boy's house and stabbed his father with it just to be amusing?"
An Eye for Detail
It's clear that #4 has a very good eye for facts and evidence. In fact, he can repeat almost every detail of the trial: "The woman saw the killing through the window of a moving elevated train. The train had five cars, and she saw it through the windows of the last two. She remembers the most insignificant details." Juror #4 is probably more likely to trust this woman's memory of details because he's the kind of person who remembers details, too.
For most of this movie, Juror #4 reaches the same conclusion as guys like Juror #3 and Juror #10. But don't let that make you think they're similar. Juror #4 is actually disgusted by the prejudice he sees in these men. He even says, after one of Juror #10's racist rants, "Now, sit down and don't open your mouth again." And when push comes to shove, he changes his mind about his verdict because he realizes he missed the fact that one of the witnesses was not wearing her glasses when she apparently saw the murder.
Instead of throwing a fit like some other guys, Juror #4 simply says, "Strange, but I didn't think about it before." And just like that, he changes his mind and does the right thing.