Four trackers are fun, but I think you'd be better off with an 8 track machine, even if that means 8 track cassette. Tascam made a few 8 track cassette machines, the best of which was the 238, but it requires an external mixer. They all made 8 track Portastudios, including the 688 (which I believe was every bit as good, in terms of being a recorder, as the 238) and the 488 and 488 mkii.
I used to own a 488 and regret that I ever sold it. I believe the fidelity is great, particularly when you consider that you're getting 8 tracks on a cassette. The advantage that the 238 and 688 have over the 488's is that they can record on all 8 tracks simultaneously, where as the 488's can record on up to 4 tracks at a time. If you are recording just yourself, then you'll probably never need to record on more than 2-4 track simultaneously anyways.
In Boston there's a small-chain of music stores called Daddy's Junky Music. Very often they have some great prices on cassette multitrackers, and the benefit of buying through them is the offer a warranty on the gear and you can purchase an additional warranty for the gear. Whenever possible I try to buy my gear from them, I've never had a bad experience with them over the past 10 years. They now sell gear online as well.
http://www.daddys.com/detail.php?itemTitle=tascam-488-porta&itemNumber=TAS0569
This is a link to a Tascam 488 that they are selling for $139 including shipping. You could always give the store a call to inquire about their equipment.
The next step up from a cassette portastudio is the mighty Tascam 388, which is an 8 track, self-contained recorder and mixer 1/4" reel to reel machine. These are excellent machines that generally sell for under $500, but I would advise only in purchasing one locally, as they are very large, extremely heavy, and likely to be damaged during shipping.
Best wishes,
-MD
). I wouldn't rule out either one but keep shopping around until you find something acceptable in your budget.