The Dragonflight Patch 10.2.6 Public Test Realm has been reactivated and partially updated for Season 4! Although item test vendors and raids have not been updated for the fated season, Mythic+ keystones changes are available for testing.
Patch 10.2.6 already went live last week, though Season 4 is also a part of the patch and has yet to be tested on the PTR. While the most recent This Week in WoW bluepost stated that the Season 4 PTR testing would begin today, it appears to be only partially rolled out, as Mythic+0 was just hotfixed over the last few minutes, from the previous 440 item level to the expected 489.
There also have not been any new public builds, but we expect further Season 4 changes to be hotfixed onto the servers as time goes on.
Update: Some previous season items are now appearing on the PvP vendors. They are only 250 item level, but can be scaled up in Mythic+ with the test aura.