You set up your mix of the first 3 tracks. Send them to track 4 and record it. Now you can erase 1 2 and 3. You have 3 open tracks to record. Its the basic bounce method.
Just remember that every time you bounce, you are adding a generation of noise. You need to make sure you have a good mix, since all the levels are set.
Trying to cover the erase head was a trick, but I always though that it degraded the previously recorded track. 50 years ago it was fun to play with, but I honestly was never really happy with the results. You tried it because all you had was a 2 channel cassette. It was much better to go to reel to reel and more tracks. But today you couldn't give me a tape deck. There's no "romance" to it for me.
The ONLY reason I have a tape deck was to play old tapes that I already had. I fed them all into my audio interface and digitized them. I did the same with my video cassettes. After having 3 Hi8 cameras go bad, I figured it was time to preserve the videos before they became irretrievable.