I've rebuilt Borg Warners and muncies, but haven't done 4-speed work in decades. I would say you need to get an exploded view of your trans.

First you will need to determine what trans it is ...early T-10, with aluminum or iron case ...or Super T-10. There should be some numbers stamped in the case which will indentify it.

If it's a Super T-10, I seem to remember they had straighter cut gears, like an M-22 Muncie, which makes them stronger but noisier. Fourth gear is just a "straight through" mode, so there is no gear noise. You will also need to know what ratio first gear is in the trans, as they vary.

Once you determine these things, you can get a drawing to determine what is damaged, and contact a parts source to see what is needed for a rebuild.

I hope this helps to get you started.