And that's #5 - over to you
The one I was hinting at was the weird version seen in A. Bertram Chandler's Commander Grimes novels, which gets up very close to light speed then does something stupid like firing the boosters to break the light barrier. As a result the ship completes its journey very fast, and also gets converted to antimatter - which is a good thing since some of the Rim Worlds of this universe are made of the stuff, it's the only way to visit them. You do it again to get turned back to ordinary matter on the return journey.
Believe it or not, this is safer than both of the previous FTL drives - the early one had a habit of ending up in entirely the wrong solar system then burning out, hence numerous lost colonies, the later one messed around with time so badly that Grimes got to visit various parallel worlds, the past, and the future. It just wasn't popular because of the whole antimatter thing.