BetaSP certainly DOES have the ability to record 4 tracks. The UVW series of decks and the PVW series of recordable camera backs only record 2 tracks, but the higher end decks and the BVV5 back WILL record 4 tracks of audio.
I’m going from memory here, but I’ve used 75s and similar decks a lot. First, on the white buttons on the left side of the front panel, make sure to press audio 1 and 2 and they will light up. Then you will also have to select how the deck records the tracks. There is a switch that’s on top of the front panel when you slide it out that says “AFM Input”. Make sure it’s set to 3/4. I think that’s it, although it’s possible you may have to flip some switch underneath the front panel when you flip it up. But I think all those switches are for time code and the like – the memory is failing me here. Then when you do your insert edit tracks 3 and 4 should record.
Hope this helps. Have fun!
Rob