gamescom? TGS? VGA? E3 2014? or just announce it not on a gaming event?
personally i think VGA would be the best time to announce it. the PS4 would probably be released by then, the initial hype strong, but starting to fade away, and then bam! naughy dog announces its new game for the ps4, it looks freaking awesome and everybody is talking about the ps4 and naughty dogs new game.