advantages
This point may be trivial, but I just wanted to throw it out there that a 4-5% advantage does not really exist in blackjack by card counting. Let's say you play the excellent Bellagio double deck, using a level-two system with full indices, and a 100:1 spread (SCORE = ~160, according to CVCX). That will still give you only a 3.1% advantage. And that's about as good as it gets.
If you play a good rules SD game like at Barona (rule of 5), it's also a 3.1% IBA. If you could somehow convince them to give you something silly like rule of 8, then yes, your edge is now 7.0%.
If you play a strip rules 6 deck game with (yikes) half a deck cut off, your edge is now 3.4%.
So as you can see, even edges in the 3% range is only possible with a combination of good rules, unbelieveable pen, and ridiculous spreads. And since you can't really maintain that win rate for long, it's really only a theoretical discussion.
So yeah, advantages in the 2 to 3% range is possible for straight counting, for short periods of time. But 4-5%, no.