The most reliable way to peer into the future is probably to look at the past. Let's look at the past seasons and how they have kept pace with PvE content:
Season 1: Jan 30 - Jun 19, 2007 (20 weeks)
The end of Season 1 coincided with patch 2.1.2, with no major content changes. Black Temple was made available for play earlier on May 22, 2007 with Patch 2.1.
Season 2: Jun 20 - Nov 26, 2007 (23 weeks)
Season 2 ended two weeks after patch 2.3, which made Zul'Aman available.
Season 3: Nov 27, 2007 - ??
If Season 3 gear is pegged to Black Temple loot, it took PvP gear five months to "catch up" since the first slaying of Illidan on Jun 5. According to WoWWiki, a 20 or 23-week run will put Season 3's end between Apr 22 and May 13. These dates are unlikely as it is PvE progression that is playing the catch-up game now.
According to party-pooper Kalgan, "S3 rewards have quite a bit of life left in them before they start to feel obsoleted by PvE gear." Drysc has also explained that PvP gear is pegged to PvE gear progression. As noted by Zach, this means that as long as high-end Sunwell Plateau loot hasn't made its way into the market, there will be no Season 4.
At the time of writing, only a handful elite guilds have cleared the Eredar Twins after the first gate opened on April 8. Patch 2.4 landed on March 25 - two weeks before the opening. Access to the final two bosses of Sunwell Plateau, M'uru and Kil'jaeden, are currently still blocked by two gates, which are slated to automatically open at equal intervals. The gate openings were originally tied to the Essence of the Immortals quest chain, but Blizzard removed this requirement a few weeks before 2.4 went live.
With presumably two weeks between the opening of each gate, we are still four weeks away from any guild attempting Kil'jaeden. If we allow another two weeks for Sunwell Plateau loot to make its presence felt in-game, this will place Season 4 sometime in the early part of June 2008, making Season 3 the longest season since the Arenas started.
The Arena Tournament is another big event on the WoW calendar, with finals slated for October 2008. (On a sidenote: is Blizzard hoping to tie the tournament final with WotLK's launch event?)
The first qualification round has started since Mar 31; the second qualification will begin on June 3 and end only on July 15. This is the crunch: an early June Season 4 might land while the second qualifying round of matches are being played out!
With prizes on the line, I doubt Blizzard will have the fortitude to shift the playing field of the tournament by introducing new gear in the middle of a qualifying round, even if they are freely available on the tournament servers. This means that tournament participants will be competing with gear that is a whole two seasons behind what is available on the game servers, all the way till October '08.
It is unlikely that Blizzard will risk taking the wind out of its own tournament sails by restricting Season 4 gear to game servers only. I also doubt that tournament participants will take kindly to playing with "outdated" gear, especially after paying $20 to compete.
To avoid stealing thunder from the tournament, Blizzard could release Season 4 before or when the second qualifying commences on June 3. In the worst-case scenario however, Season 4 may only start sometime after the end of the qualifiers on July 15. This will stretch Season 3 to more than 30 weeks! Can we wait that long?
Update: Originally slated to end on April 14, the practice phase of the qualifier has been extended to April 22. Registration is still open and will only close when the final phase of this round kicks in on May 6.
Note: Edited to correct mistake. Apologies and thanks Shefki.
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