It needs little silver stickers attached to the cassette provided by Yamaha.
Here is how to bypass that "feature". I just opened mine up and found a mod that the original owner did.
On the main internal circuit board just north of the blue transport control buttons are two solder through holes. They are labeled "4 ch fix" with an arrow pointing to the holes. Solder two wires in these holes and run to the back panel. Add a plain mini toggle switch. You can now select 2 track or 4 track mode at will....bypassing the need for sensor stickers/seals.
The deck operates in 2 track by default...but a jumper between these holes locks it in 4 track mode. Yamaha hid this on the inside probably for test purposes.
The Machine:
The Bypass:
The Rear Switch Added:
Enjoy! Hope this helps all the people that have been looking for this answer through the years.