It depends on whether you want a big piece or just a filler, not to mention how fast you think you can learn something more difficult. Can you be more specific?
Since it's just four pieces, I figure something along the lines of pretty substantial. I may ultimately want to use it for my graduate school auditions too.
L'isle Joyeuse; Pour le Piano; Images Bk 1 or 2; set of preludes (3 or 4 ~ 10-12 minutes);
L'Isle Joyeuse is only five minutes long.