When you say that you have recorded four tracks, I assume that you mean four audio tracks, all for the same song, not four individual songs, also sometimes called "tracks". If this is the case, you "mixed" the four recorded tracks into a stero recording that you are now wanting to re-mix. This being true, you should still have the original four tracks somewhere, unless you recorded over them (if using tape) or deleted the files (if using a computer).
If you don't have the original tracks or files, you can't do anything more than tweak the existing stereo mix by using effects (EQ, compression, reverb, etc.).
All it takes to re-mix the tracks is to pop in your tape (or load your file) and play it, adjusting the faders and trim and EQ just like you originally did in the first place to get the original mix.
It's just that simple.
May I recommend that you buy one of the MANY books available about home recording and read it from cover-to-cover two or three times. I've heard that anything written by Craig Anderton is great. I started with one of his books in the early 90s....