How to get 4 channel audio from a MacMini?

I would like to know if/how is possible to get 4 channel audio from a Mac Mini.

My speakers do have two jacks for from/back and currently I can use one one of them.

It seams that putting the second jack into the line in doesn't work (some Windows audio boards do let you configure the purpose of the jack socket.

Is there any solution or should I get used to stereo?

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.

Receivers are combination devices that do several things in one box, like tuning, amplification, and switching, but you don't need most of that just to decode a multichannel digital signal.

What you're looking for is a surround sound decoder. Here's one for $60. I don't know how good it is, but it's far cheaper than any surround receiver I've ever seen.