re: Video blackjack question
Card counting is not meaningless in a 6D game with an unannounced shuffle after 4D, but it is largely ineffective. The reason is as you stated: the shuffle point is unknown.
If you start with a count of 0 when you begin play, then reset to 0 after every four decks, it will be worse than if you're playing against a 6D shoe with a known shuffle after 3D. Here's why: from a penetration standpoint, the best scenario is if the cards are shuffled very close to the point when you arrive. If they're shuffled shortly before or shortly after you arrive, it's as if you're playing with a shuffle point of close to 4D. From a penetration standpoint, the worst scenario if if they're shuffled 2D after you arrive. Here, it's as if you're playing against a shuffle point of 2D. The average of these cases is 3D, providing a very rough estimate.
There's another factor to consider: from an accuracy standpoint, the worst scenario is if the cards are shuffled after you've been counting for nearly 4D and your count is high. You'll be playing and betting as though there's a high TC, when in reality the TC is nearly 0, since the cards have been shuffled recently. You could potentially make serious betting and strategy errors here. Calling your count in this game a TC is misleading, since there's nothing "true" about it; it's based on unjustified assumptions about when the cards were shuffled.
In short, a 6D game with an unknown shuffle after 4D have been dealt is not worth playing. If you insist on playing it, however, a better approach might be to reset your count to 0 after every 3D, avoiding some of the costly errors that could come near the end of a 4D span without a reset.