iMac Intel 20" & Altec Lansing 641

First off, I had recently acquired a new 20" iMac Intel today at my local store and couldn't be more pleased with it so far!! Now the question. I have been using my old Altec Lansing 4.1 speaker system for years now and has never let me down and would like to continue to use it. I noticed that the iMac has just 1 audio out connection. The Altec Lansing 641 speaker system is a 4.1 system and requires 2 audio connections, the front and rear channels. In my previous PC system, it had a Sound Blaster Audigy 2 card so it had specific jacks for front and rear channels. I was able to hook up just the front channels and it works just fine, however I did notice that the jack sticks out a little(pic below). Also, can I just hook up a splitter to utilize my rear speakers as well?