With all the new models and data files showing Season 4 Arena gear, it’s easy to assume that season 4 will begin when Patch 2.4 goes live. Kalgan however, appeared on the forums recently to debunk the thought.

I’ll be slightly more descriptive than Drysc. Not very soon. S3 rewards have quite a bit of life [...]

They’re not confirmed yet and only based on information found in the game files. (They should be fairly accurate though)

Here’s your 1st look folks! I put out some screens a while back that I’ll repost again. The quality is amazing and I am extremely happy to provide you all with these screens. As more information comes out I’ll post updates.
Brutal Gladiator’s Greatsword (2H Sword)

Brutal Gladiator’s Vindication  Paladin Retribution set
       

The non-set [...]

Don’t freak out, there’s no official word from Blizzard regarding Season 4 starting with the release of Patch 2.4 but it is being speculated. Otherwise why would these by on the PTR files?
Brutal Galdiator’s Battlegear (Warrior)

Druid Resto, Balance, and Feral breastplates.
   
Holy Paladin breastplate.