The RC-5 is really a different knife and somewhat unique. It was my first RAT (it was still RAT when I bought it). I didn't have anything in my collection that was near as beefy or bullet proof and since I do fly, its my companion in the cockpit of my little plane, strapped to my right leg. I know I could cut through aluminum or plexiglass with it and still have a great survival tool. However, for every day bushcraft, hiking, backpacking, etc. the 3 or 4 is better. I like how the 3 handles the best for its handle size, but the 4 gives more versatility. While I started with the 5, I picked up the 3 and 4 shortly thereafter. You will probably do the same.
After all that, I also say 4!