#1
My music teacher at school has put me and 3 other people in a guitar quartet at school, we all play classical guitar and are looking for some songs to play (we're usually forced to play them in a school concert ). It doesn't have to be classical music, it just needs to be for acoustic guitar, and it needs 4 guitar parts...or we could possibly improvise with 3.

2 of the people aren't exactly great at guitar (not trying to sound like i'm better than them ), so it can't be anything really fast..but yeah..any suggestions?! thanks
#2
try les yeux noir by django reinhardt, its 2 guitars, but you could double up the rhythem and the solo like its normally played.
#3
Maybe you could take Vivaldi's Spring from The Four Seasons and work out something with that, have two of the guitarists play the main melody, one of the slower guitarists play the underlying harpsichord (it's just quarter notes), and the other slow one play backing chords or something.

Or are you not allowed to arrange non-guitar music for guitar? That was just the first thing that popped into my head..