Okay, the 414 records at a higher speed than standard tape decks play back, for one thing. Second, the way the 4143 works is as follows:
You can only use one side of the tape because the Tascam 414 treats the entire tape as one side. Think of the tape as having 4 separated "channels" spanning equal widths of the tape. In a normal tape cassette player, one "side" of the tape is one set of those 2 channels, and the other "side" is the other 2.
If you were to record onto four channels of the 414, you would then find when playing the tape in a normal player, that on Side A of the tape, only tracks 1 and 2 play back, but 3 and 4 disappear. If you flip the tape over, 3 & 4 are playing backwards.
What you need to do is run the stereo output from the 414 to another recording device, that records normal stereo, be it into your computer, or to another tape recorder that records to one "side" os a standard cassette. The 414 should have main outs Left and Right of some kind for this. Hope that helps - if its too confusing, feel free to ask away!