It's not clear what type of setup you are looking for:
1. 2 paralel stereo, or
2. Surround sound
You fail to mention the Sound Card (on board?) and the types of speakers -
self-amplified, digital etc.
For 1:
To connect all four, you need a "Y-splitter" - available at electronics
stores or at Internet sores; Radio Shack sells them for $3 or so. There's
another solution using a speaker hub that connects to the sound card and
sits on your desk; here, you can switch speakers, headphones, mic. etc.
There are many types of Y splitters - you probably need a "mini-jack to two
mini-jacks (might use 2 RCA, though)." That is, whatever sound card output
to whatever speakers inputs, extension cables to speakers etc.
For 2.
you can do this from the sound card or from an amplifier:
make up your mind what you want the extra pair of speakers to do: rear
speakers in a 4.1 setup, for ex. Then, hook up the new speakers in the
appropiate port and adjust speaker settings via "Speaker Settings" applet in
MS Sound or whatever.
Michael