Best t8 motor for 4s?

[RC-Monster Mike]

I don't think anybody can say this as a factual statement - it is a matter of opinion. "The best" for one user may not be for another. Some like aggressive power, while others need smoother delivery. The most ideal for one track, user or driver style may not be the best for another.
Personally, I like the 1900Kv buggy combo in a 1/8 buggy. It runs equally well on 4s or 5s, runs cool and is very responsive to timing changes. Is this "the best performance"? - for some folks maybe. :)