Philosophically, Blizzard wants to see more guilds kill Kil'jaeden before Season 4 begins. Considering the pace of the playing community -- a mere 3.88% of the guilds worldwide are even in Sunwell Plateau and only .03% have killed Kil'jaeden -- it is likely that it will take a while before we see people running around in Brutal Gladiator gear. With a mere fraction of the playing community able to step into Sunwell, and with progressive guilds breaking apart left and right, the prospect of Sunwell-tier gear spreading into the player base quickly is rather small.
This is not really news. Blizzard has mentioned the symbiotic relationship of PvE progression and Arena seasons before, stating that "S3 rewards have quite a bit of life left in them before they start to feel obsoleted by pve gear." The surprise lies in actually how long it would take. The first Arena season, the rewards of which are designed to match Tier 4 gear, lasted 20 weeks; Season 2, which is a match for PvE Tier 5, lasted slightly longer at 23 weeks. The current Arena season, with Mount Hyjal and Black Temple level gear, has run for 28 weeks. Considering that Blizzard gives two weeks notice prior to the beginning of the Season, this means that Season 4 will last at least 30 weeks, two full months longer than any of the past seasons.
Season 3 began about six months after the release of the Black Temple. If the same grace period were to be applied for Sunwell Plateau gear to be distributed to the player base -- starting from when the last gate was opened, it means that Season 4 could possibly begin on November! With no new content outside of Wrath of the Lich King on the horizon, this could mean a long period of stale content.
On the other hand, Season 4 introduces personal ratings requirements to most of the Arena rewards, creating a distribution control mechanic that wasn't present in the past seasons (with the exception of Season 3 weapons and shoulder pieces). The long delay between the release of the Black Temple and Season 3 didn't matter so much because many players were able to obtain Season 3 gear the same week it started, closing the gear gap between PvE and PvP in a short span of time. With personal ratings required for most pieces, Blizzard guarantees limited distribution of Season 4 rewards. This means that the time between the release of Sunwell Plateau (or the opening of the last gate) and the start of Season 4 should not be as long as the delay between Black Temple and Season 3.
In the past, so-called welfare epics were accessible to the entire player base because of the low barrier of entry to Arena play. This allowed virtually any player to acquire a full set of Arena gear while it was considerably more difficult for players to acquire equivalent PvE gear. It is easier, for example, for players to acquire 5/5 Merciless Gladiator gear than it is for them to get 5/5 Tier 5. The long period between PvE raid releases and the start of Arena seasons was essentially a control mechanic that gave raiders a head start before the rest of the player base caught up quickly.
The new personal ratings requirements, however, create an separate progression for PvP. Because even Honor-bought Season 4, or Guardian, gear require personal Arena ratings, players will be required to achieve a certain level of play beyond simply spending time in the Battlegrounds or Arenas. Players must not only invest time into PvP but also raise their level of play to a certain extent. The demands aren't extremely high -- all pieces except for weapons and shoulders can be obtained upon reaching a personal rating of 1700 -- but it requires a particular dedication to performing well.
This means that Season 4 will not see the kind of item flood that happened in the past. In fact, with the stringent requirements to weapons, which require a PR of 2050; and the shoulder pieces, which require a restrictive 2200, the amount of people with complete Season 4 rewards will be extremely small. Roughly the same amount of people who've killed Kil'jaeden, in all likelihood.
The only apprehension is that the player penetration into Sunwell Plateau might never reach huge numbers. Despite over a year, only 20% of guilds have even set foot into the Black Temple, with a mere 6.9% of players having downed Illidan. What is the threshold that Blizzard is seeking for Sunwell gear distribution? The numbers for Sunwell Plateau promises to be considerably less than 20%. Blizzard has admitted to learning lessons in raid accessibility, prompting them to create 10-man versions of 25-man content. How long will it take for Blizzard to decide that enough Sunwell Plateau gear has spread out into the player base before they start Season 4? With the new mechanics, at least, it shouldn't take until November.