There are two issues here.

The first is that you are attempting to stretch out a track to four times its original length!  That is asking a lot.

Secondly, the audio you have recorded to your computer is a digital approximation of the original analog track.

http://en.wikipedia.org/wiki/Digital_audio

It is sampled at the set sample rate, which is usually 44.1 kHz.

http://en.wikipedia.org/wiki/Sampling_(signal_processing)

Assuming you can do a single time-stretch to achieve your goal, you are making another approximation of an approximation. It is stretched and re-sampled once again at the set sample rate.  The more you stretch it, the worse it will be.

The more you do this the more errors will be created.  It's like making a zoomed copy of a zoomed copy of a copy on a Xerox machine.  Eventually, the document will be so bad it will be almost useless.  The same principal applies here.

In Magix Music Maker, you are only able to time-stretch to double, or half, of the original object time.  To get it four times the length, you would have to do this twice - in effect creating a copy of a copy of a copy.  The results are not good.

And, that's using a WAV file.  If you are using an inferior format, such as an MP3, the results will no doubt be even worse.  If you were able to record and then time-stretch at a 96 kHz sample rate, the results *might* be better.

I'd recommend trying to find a tape deck that can play the tape at the correct speed.  I realize that may be next to impossible in this day and age.