Grievous returns in next Friday's all-new episode of The Clone Wars as is evident from the following synopsis at the SW.com episode guide for "Shadow Warrior" (S4 Ep. 4):
"When the leader of the Gungans, Boss Lyonie, is injured it's discovered there is an uncanny resemblance between the Boss and Jar Jar Binks. Binks must sway his people from rising against the Naboo and stopping a Separatist invasion led by General Grievous."
Click here to check out the full guide which includes a trailer, screenshots, concept art, and the opening tag line.