Series premieres for anything, usually grab a nice handful of viewers, given their advertising was good. 3.5-4mil viewers for a premiere is pretty average, but rather exceptional for a network like Nicktoons (its highest-rated series premiere was X-Men before DBZKai, and its highest-rated show overall was a series finale of Avatar, which garnered over 5 million viewers).
As far as anime on television, this right here, makes Dragon Ball Z Kai the highest-rated anime being aired on American television.
It's destined to climb. Given the new audiences, and whatnot. Dragon Ball took off slow over a decade ago, but eventually became one of the highest-rated animated programs in American history. Just like I said, with a new audience and millions of previous fans, it's likely DBZKai could reach some astounding levels on Nicktoons, as the series picks up.