Eyonix posted this on the official forums:

We’re currently planning to end the third Arena season and begin Arena Season 4 on June 24, 2008. This exciting Arena season will introduce the Brutal Gladiator set, and will also allow players to purchase Arena Season 2 items using the honor system. Please note that we will be resetting all Arena team and personal ratings. Players will still retain their Arena points and teams with this reset. The team and personal rating will simply be reset to the default 1500, allowing all teams to once again compete for top honors with a fresh start.

Also, with the end of the third season, players on the top teams from each battlegroup will receive their end-of-season rewards. These include Arena-specific titles that they can display proudly until the end of the new season, and, for the best of the best, an Armored Nether Drake. Please also be aware that with the end of the current season, all previous end-of-season titles will be removed.

Does this mean we'll see Season 5 around WotLK? We'll just have to wait and see! For discussion on arena goings on head over to our WoW forums!  For a look at the Season 4 gear try this!