Why not do 6 "performing" pieces. I.e. playing guitar AND singing. Would that be easier for you?
I just did my LC music, so i think i have some authority to comment. Basically, if you were doing 6 guitar pieces, you would be looking to show off 3 different styles of guitar. e.g. classical, blues, rock, general accompanying, jazz. Since you're doing 4 pieces, i'd advise 2 "styles".
You'd need one complex song (preferably two) one contrasting piece, and one piece you know so well you can play it backwards while being upside down. You'd start off with the piece you know backwards, then do a complex one, then the contrasting then complex again.
System of a down- Roulette
This would have to be your "Easy" piece. Its a bit basic, and so should be the easist of all your pieces. Everything else should be harder than this.
Under the Bridge:
Good choice, so long as you can get the strum/picking worked out in the verse, you're flying. Its a good piece, but you'd have to sing it aswell, otherwise it could get boring. This is your "rock" piece. (also a complex piece i suppose
Jimi Hendrix- castles made of sand:
Introduce this as a blues style piece, and this would definately be one of your "complex" songs.
Last song, i'd advise doing a nice classical piece, such as "sarabande" by bach. Its a grade 6 piece, so you should be able to pick it up easily in any music shop. This can be the contrasting piece.