Anna, I can say that I pretty much agree with the thought that a 4 track digital would be better in some ways, many actually. With that in mind, and, I think, understanding where you're coming from, go ahead and run with the cassette rig for awhile, particularly since you now have access to a unit without an monetary outlay. If you were to need to make the purchase of a new unit I would certainly agree whole-hearedly with Substudio but not where you are now.
As I mentioned before, I use a cassette recorder for lots of work. That's because it is a very good unit and I have it, not because it's the best thing to use. It also did not cost me anything. I too like the recordings done on tape and it's my feeling that, even with the slow cassette speed, tape just sounds better than any digital unless the digital has been tweaked.
Stay the course and see how you like it. If it works and you like it you can then make the decision as to spending the money. If you are then absolutely stuck on tape I'd look into a reel to reel machine, even 1/4 would suffice for 4 tracks. Again the looming and riding cost of tape has to be taken into consideration.
Lot of stuff to think about for you, and to try. Then make the decision.

As to the guitar, mic it, don't plug direct.

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