Bouncing between tracks on Porta 03

[porta03]

I just got a Porta 03 for demo purposes. This is my first cassette portastudio and I am looking for info on whether it is possible to bounce between tracks on this machine. What I have tried is recording on tracks 1-3, then panning them all left and plugging the left output into the line in on track 4. Track 4 is turned down all the way on the mixer, but I'm still getting squealing feedback. Am I doing something wrong, or is this just a limitation of the machine? Thanks.