My personal opinion is that if you are pairing your RC-X with another knife, axe or chopper, it makes a lot of sense to get an RC-3 which is best optimized for slicing. That 0.125" thickness and the fact that it comes with a steeper bevel edge angle (30o total edge compared to 40oC total edge for RC-4) will allow it to easily outperform the RC-4 in cutting tasks. In the RC-3 you will also have a more optimized EDC knife that is lighter and you get the molle-back included with the knife whereas this is an extra cost option for the RC-4. Paired with other RC- offerings, the RC-3 and RC-6 should make an excellent combination.
On its own, or as a survival knife with a possible fear of the condition where you're other equipment gets lost, I think the RC-4 is a better overall performer. More robust, nicer blade length for batoning (although the RC-6 has it beat here), it truly is a survival blade on its own.
Admittedly, I completely abondoned this logic and bought an RC-4 and now have an RC-6 on pre-order. I won't likely ever carry these two knives together, unless my wife takes the RC-4 and I take the RC-6 on a given vacation. They were purchased in mind to take on different types of trips where I envision using a smaller or larger knife. I anticipate that the RC-4 will see alot more duty than the RC-6 over the long term.
I'd like to get a tiny knife to go with the RC-4 and/or RC-6. Had I gotten the RC-6 first I would have probably then opted for the RC-3 right away. I'm thinking something like the tiger knapp (thanks RAT for endorsing them) with custom kydex neck sheath will do the trick.