Alright so I'm gonna throw something out a bit different here:
Passive damage weakness: Good While it is not actually weak to anything, it is also hit by everything.
Good Overall STAB combo:Good This is really the area where this typing might be too good. Electric and Dark have almost perfect neutral coverage for OU pokemon. That being said the combo is resisted by a small handful of pokemon, all of whom would probably have a good shot at using CAP4 as setup bait.
STAB that cannot realistically be spammed endlessly for maximum impact:Okay Well like I said before the STABs are not resisted by a huge number of pokemon. However since in my head this category means "CAP4 is currently choice locked, how much negative impact is this gonna have on it", I would say that neither of these types works amazingly either.
Lots of Weaknesses and Resistances:Good This typing comes with 5 resists, 1 immunity, and 3 weaknesses. While at first glance this seems to be eliminating risk, a closer look may say otherwise:
x.5: Electric, Flying, Ghost, Dark, Steel
x1.5: Fighting, Ground, Bug
None of the resistances are very common attacking types, but it does give CAP4 a good selection of potential switch ins. The weaknesses, on the other hand, are all fairly common attacking types, so this would force CAP4 to be careful about when it came in.
STAB on notable risky moves:Okay On the Dark side it would have the Pursuit/Sucker Punch combo, and on the Electric side it has Wild Charge and Thunder. While this is not the best selection of risky moves, they are there.
The biggest check against this is that the STAB coverage might be too good, but I think the defensive weaknesses balance the offensive pluses.