Quote 4
BANQUO
You should be women,
And yet your beards forbid me to interpret
That you are so. (1.3.47-49)
"Should" be: why? Because they look like women, or because they're obviously supernatural? And does the presence of a beard automatically disqualify someone from being a woman? (Don't tell the moustache-bleaching industry.)
Quote 5
BANQUO
That look not like th' inhabitants o' th' Earth
And yet are on 't?—Live you? Or are you aught
That man may question?[…]
You should be women,
And yet your beards forbid me to interpret
That you are so. (1.3.42-44;47-49)
If Macbeth were a horror movie—which it kind of is—then Banquo would be the skeptic who gets killed because he refuses to believe. Where Macbeth accepts the supernatural unquestioningly, doing some pretty dumb things like following a floating dagger and arguing publically with a ghost, Banquo isn't to completely discard his reason and rationality. Unfortunately, that turns out to be the wrong choice.
Quote 6
BANQUO
If you can look into the seeds of time
And say which grain will grow and which will not,
Speak, then, to me, who neither beg nor fear
Your favors nor your hate. (1.3.61-64)
Let's assume that the witches are actually supernatural beings. (Just go with it.) Banquo is showing us how to approach the supernatural: very carefully. He doesn't want any favors from them, and he's not afraid of ticking them off. Although, considering how they feel about chestnuts, maybe he should be a little more cautious.