Your Mac Mini has optical audio-out, which supports multiple channels.
Apple has cleverly put a little optical transmitter inside the headphone jack (many Macs have this, not just the Mini). To turn it into something useful, you have to use a little adapter (like this one to get a Toslink signal.
From there, it gets interesting.
You say that your speakers have to jacks. I assume you mean two analog 3.5mm (headphone) jacks.
Unless your speakers have a digital or optical input, you'll need to get a receiver that can do the digital-> analog conversion. Something like this should work (there are tons of options, this is just the first one I found).
The key is to look for something with digital/optical in, and 4+ channel preamp out.
Another option: The Mac Mini has HDMI out. You can find a receiver that has HDMI input (like the one above), and 4+ channel preamp out.