The Hollywood Reporter has an article saying that with Glen Mazzara leaving The Walking Dead after season 3, current executive producer Scott Gimple will take over as showrunner.

Gimple joined the series based on the comics created by Robert Kirkman during season two and penned the third and seventh episodes, co-writing the 10th episode with Mazzara. He wrote the sixth and upcoming penultimate episodes of season three.

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Mazzara will remain on board as showrunner through the second half of season three, overseeing postproduction on the series, with filming already having been completed on the final eight episodes of the ratings juggernaut. Gimple will take over when Walking Dead resumes production later this year for season four.
Source: The Hollywood Reporter