Although we've speculated in the past as to when Arena Season 4 will be going live, our anticipation fanned by the revelation of more and more Brutal Gladiator gear, Kalgan pops by the WoW General discussion forums to pour cold water on everyone's expectations by saying that Season 4 will start "Not very soon." He explains further that "S3 rewards have quite a bit of life left in them before they start to feel obsoleted by pve gear." Given that the Arena items uncovered so far in the PTR are only a fraction of what is supposed to be a wide collection, this sounds more than plausible.
As Drysc explained earlier in the same post, PvP gear is designed to match PvE gear progression -- for instance, Arena Season 3 items are roughly on par with Black Temple and Mount Hyjal gear. The Sunwell Plateau needs to be unlocked in stages, as quests and raid drops open up more content, meaning most of the PvE gear that is a step up from BT/MH gear won't be available until at least a few weeks after Patch 2.4 goes live. It stands to reason that Arena Season 4 will only begin when most of the PvE progression has access to the Sunwell Plateau's latter bosses such as M'uru and Kil'jaeden. Considering that Patch 2.4 doesn't seem quite done yet, Season 4 is probably quite a ways off.