You can't do 5.1 with 4.1 speakers, mate...
Basically, 5.1 does front left, front right, left, right and back (as well as a sub), whereas 4.1 does front left, front right, back left and back right (with a sub). the ".1" refers to the sub.
Basically, set it up as a four speaker system.
Apple? Macintosh? What are these strange words you speak?