Yo Luth from DeLUTH:
If you are mixing down from a digital 4 track to minidisc, you will lose sound quality.
If it is a 4 track analog box, you might gain some quality.
How to do it?
If your 4 track has 4 outs, you could run them into an MD8 track mini disc, and then you can mix the sounds down to a tape deck or a DAT, or a PC I imagine.
I have an MD8 so I'm fairly familiar with that box. But, now I'm using a Yamaha 2816 and won't be using the MD8 much longer.
You can do a couple of things. You can buy an MD8 minidisc which will give you 8 tracks to record and you can mix down to whatever.
Or, you can buy a DAW box and do it in the digital domain. Or, you can do what you asked, mix down 4 [unless you bounce tracks and lose control] tracks to whatever fits.
Hope this gives you some insights.
Have fun and Happy St. Patrick's Day
Green Hornet