User blog comment:Master Sima Yi/Assassinews 03-03-'13 — Assassin's Creed IV: Black Flag trailer and screenshots/@comment-4999424-20130322171533/@comment-4868246-20130324002410

Even if Adrian (Haytham's voice actor) wasn't aware of any involvement in future installments. This doesn't mean he's not coming back because typically voice actors do most of their work in april to july or so according to the Assassin's Den.

Anyway, I just wanted to point out that voice actors don't start voicing until most of the script has been composed :)