Hmmmm... 10 tracks on a Portastudio...
Well, the math works out, but I would'nt take the "reduction" method of bouncing too far.
There are obvious compromises in sonic quality when you bounce tracks.
Some bouncing is ok, and a real expert may be able to get a 'robust' sounding 10 track recording on a 4 track format, but most lay-people out there with Portastudios -can't-.
More likely, they end up with 10 'virtual' tracks that sound overproduced, yet still weak , with noticeable hiss, and some parts so weak that they're lost in the 'mud'. I'm not saying it's impossible to get a reasonable sounding tape by using this technique, but don't get too optimistic about the possibilities of bouncing tracks on your Portastudio. It works, but usually with a price in sound quality to pay.
Don't get me wrong, I bounce a few tracks occasionally, but I would not try to drive it to that extreme (10 reduced tracks on a 4 track). I've done it, and it was not good. Better to get an 8 track or larger multitrack tape recorder.
Now, the Porta02 should be able to mix down to stereo, with stereo output present on the 'line out' jacks, if you're cabled and configure for mixdown correctly.
The 'monitor' or 'cue' output is always mono. LINE OUT is stereo.