4,4 vs 5 or 6
I was waiting for someone more qualified to answer, but that hasn't happened yet.
My reading of Wong's PBJ indicates that in a multi-deck, DAS game you would split whenever TC=>-1 vs 5, and =>-2 vs 6; doubling <u>never</u> becomes a better option.
In general, the higher the count the more often you split pairs.
Numbers are a little different for single deck.