Out of all the abilities discussed so far, the only one I really hate is Regenerator - it really does reduce switch-in risk. I totally understand why Yllnath suggested it - CAP4 is going to have a pretty difficult time fulfilling the 'High Reward' aspect of its concept without a fairly awesome ability. In order to maintain the risk of switching in, this awesome ability should be offensively geared.
Some offensive abilities have already been suggested and here are the reasons I don't think CAP 4 should have one of these:
Download - Promotes CAP4 slapping on a Choice Scarf and acting a lot like Genesect with Bug STAB and 3 coverage moves
Rattled - A simple 1 stage speed boost in return for being hit by a Bug/Dark/Ghost attack? Not remotely powerful enough. Also, CAP4 is already weak to those attacks, and if it is taking entry damage switching in, it needs to avoid supereffective attacks at all costs!
Hustle - Only one pokemon is really at all effective with this and that's Durant. Even with great Attack and Speed, it still is only mid/low UU. The only boosting moves that pair well with Hustle are Coil and Hone Claws, neither of which are that powerful. This just isn't a powerful enough ability for CAP4.
No Guard - Reduces risk, not many low accuracy STAB moves to abuse it with. Now if CAP4 was Fire/Electric, we'd have a winner!
Guts/Flare Boost/Toxic Boost - The negative effect of HP loss on this already SR weak 'mon is not going to be encouraging it as much of a threat.
Some truly great offensive abilities:
Tinted Lens, Serene Grace, Pure Power, Huge Power, Adaptability, Contrary, Mold Breaker, Sheer Force, Technician, Speed Boost, Simple
Any of these could be good, but my favourites are Technician, Sheer Force and Simple
Technician - This would allow 90BP Hidden Powers, giving CAP4 strong special coverage and allowing us to focus on giving it the key Physical moves that it'll need in order to threaten all of OU. It also boosts Bug Bite to 90BP making it the best Bug STAB next to Megahorn (oh, and Attack Order) and Silver Wind to 90BP making a viable if risky alternative to Bug Buzz. Pin Missile also has the possibility to do major damage (max 105BP) but it's still not as good as Megahorn. Bone Rush, Bullet Seed, Icicle Spear and Rock Blast could all be added if we wanted to give CAP4 strong coverage for any of these types. Plus of course, should we give CAP4 any priority moves, these will be as powerful as if they had STAB. All in all, a solid offensive Ability.
Sheer Force - This would also enable us to give CAP4 very strong Special coverage, so could be complemented by another, more Physical Ability or a large Attack stat. Of course it does boost Zen Headbutt to a respectable 104BP. It would boost Bug Buzz and Psychic to 117BP, and coverage moves such as Thunderbolt, Ice Beam and Flamethrower would be significantly boosted also. The lack of Life Orb recoil would be very welcome and would allow CAP4 to keep switching between moves.
Simple - This would require us to give CAP4 the boosting moves to abuse Simple with, and would increase the risk posed by Intimidate users and attacks such as Crunch that would have a 40% chance to lower CAP4's defence. Still, can you imagine our little PsychoBug boosting up to 6x SpA in one turn with Tail Glow? A sort of Special Belly Drum without the HP drop. Don't worry, Bullet Punch Scizor and Scarfers would ensure this wasn't GG.
If I haven't seen further, it's because giants were standing on my shoulders...