It comes down to what you intend on doing with the knife, and personal preference. The 4 & 5 have entirely different designs. The 4 is your typical Rat design, moderately thick with a flat grind... while the 5 is SUPER thick and heavy with a saber grind, and slightly longer than the 4. I honestly think the smaller RC-4 is a better choice for most typical tasks, but... if you think you'll be doing some heavier tasks like chopping or batoning, the 5 is probably better.
Regardless, you can't go wrong with either knife.