I had a similar problem. The tonneau cover latches near the front of the truck would not unlatch, leaving me unable to take off the cover. I did not replace the cable to fix it, I fixed it with zip ties so to completely remove the cover I have to climb under the cover and pull the zip ties to unlock the latches. The latch mechanisms are held in place with a piece of metal that moves horizontally. If you look at a working latch you can figure out what I'm talking about. Lubricate the latches and then place a zip tie on the metal piece that toggles the lock so you can pull it easily and unlock the cover.
Another reason your latches might not work is that a little plastic piece that maintains tension in the cable is broken. The plastic piece is inside the lock mechanism, and you can access it by unscrewing the screws that hold the metal case over the latch. Look at an undamaged lock mechanism (one of your four should still be ok), and find a way to secure the plastic piece where it is supposed to go.