I used to have a Yamaha 4 track cassette recorder on which I did multi-tracking. It was a Yamaha MT3X. Once I had got everything recorded, I would do a mixdown onto a separate cassette deck. It was a Rotel of some sort, but I can't recall exactly now. To make a heap of cassettes means copying this mixdown, and garww above has posted a picture of a nice dual cassette.
Doing it this way keeps the whole thing in the analog realm, and thus you avoid the complexities of getting into computers. But it does require the hardware, i.e. cassette decks.
If you do decide to go with computers, rather than doing the mix in the computer and loading up the four tracks in two pairs and running speed drifting issues as bouldersound guy noted, you could more simply mix the four tracks two a stereo track which you then record as a stereo track into your computer. There are many programs that will do this. You can then do some processing on the mix.
To get it onto cassette, you can go back the other way, i.e. playing from the computer, recording onto cassette on your four-track (recording to just tracks 1 & 2). However, that depends on whether the four-track can do the standard 4.76 cm/s speed. (My MT3X was switchable between 4.76 and 9.5.)