I'm gonna go ahead and propose Download and Regenerator.
I feel they both suit the concept very well. Download has the inherent risk of switching into the wrong stat boost. It's perhaps not a big risk, but if you combine said ability with somewhat less convincing stat boosting moves, then you're ability to sweep may be more reliant on netting the right Download boost than just getting in and press the almighty Shell Smash or x Dance button.
Regenerator is where I personally feel the magic really happens.
CAP4 takes 25% damage from SR, which, in a CAP4 plagued metagame (playtest) will most likely always be up. Even if we don't think of CAP4, SR is up most of the time. Add in some additional damage like Spikes, Sandstorm, the fact that you take a hit, albeit hopefully resisted when switching in and you're taking a good chunk of health from switching in every time. This makes a choice set much more unlikely, even though Choice sets, with proper stats, can net great results in crucial moments.
By giving CAP4 Regenerator, you give CAP4 access to a range of choiced sets that would otherwise be too risky. Also, Regen CAP4 is still a risk to switch in. After you switch in once, you can bet on it that the next time switching in, you may not be switching in on a resisted hit. The risk of getting will likely increase over time as your opponent learns. The reward then is that once in, you can do some scarf revenge killing or release some banded/specced fury, and still live to switch in another time due to Regenerator.