Arena season 4 (you can click on it to see a bigger version), and here is the priest's helm from season 4. As the story goes over at World of Raids, a player on EU Arathor accidentally deleted some season 1 items, and a GM then accidentally gave him season 4 items in replacement instead. The items were taken right back (duh), but their IDs were activated on that realm, so anyone with a little item ID knowledge could pull up the stats.
For comparison, these have boosts almost all around from the season 3 items, which is pretty much to be expected. And while the Stamina is clearly much higher than anything we've seen at the higher levels, the actual damage doesn't compare much to even the Tier 6 stuff. PvP gear in season 4 is all about staying alive -- we've seen a few other items leak out already, and it's the same deal on those.
Which is probably as it should be. We should note, also, that this stuff may not actually be done -- although the fact that it's spawnable on the live realms might mean that Blizzard is ready for season 4. I still don't think we'll see it until after the arena tourney, but who knows?