In today's Patch 10.2.6 announcement, Blizzard confirmed that Dragonflight Season 4 will only launch «several weeks» into the patch cycle.
This means that Season 4 will likely be released around late April. Once Patch 10.2.6 goes live on March 19th, we will have a Season 4 PTR cycle, as previewed by Executive Producer and Vice President for WoW Holly Longdale.
Dragonflight Season 4 Mythic+ Updates
Mark your calendar for March 19 when the 10.2.6 content update goes live with a bold, new, limited-time event. You won't want to miss out on the fun. It's important to note that
Dragonflight season 4 will go live several weeks after the release of the 10.2.6 content update.
Holly Longdale
A finer point, but the update will also include everything that’s needed for Dragonflight Season 4 but will be hidden. Around (after) the launch of the update we will make Season 4 available for testing on a 10.2.6 PTR before it goes live.
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