Arena Season 4 Ends On 3.0.

UPDATE 2:

Season 4 will end on the 14th of October.

UPDATE:

I just read on Green Armadillo's Player vs Developer blog that Honour won't be reset after all:

We had hoped that the introduction of some new Honor rewards in the upcoming patch would give players a great opportunity to spend any of their remaining Honor. However, many players would still be surprised by an Honor and token reset as it isn’t something we’ve done in the past. As such, we've decided to not reset Honor or tokens upon the release of Wrath of the Lich King. Players who save their Honor will be able to spend it on level 80 rewards, although those rewards will be significantly more expensive than their level 70 counterparts. (Source)

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I'm sure that this news from Eyonix will be of interest to some:

We've revaluated certain aspects of how the arena system and current arena season will be affected during both the next content patch, and the launch of Wrath of the Lich King. For starters, the current arena season will end in respect to the ladder upon the release of our next major patch. What this means is, we'll be taking a snapshot of the current season's ladder just before the realms are taken down for patch maintenance, and our normal process will commence to determine who is eligible for the end-of-season rewards. Teams and ratings will not be wiped at that time, and will carry forward allowing players to earn points each week (and buy gear) until the launch of the expansion on November 13th. Almost immediately before Wrath of the Lich King releases, all arena teams as well as ratings will be wiped and arena charter NPCs disabled. (Source)

To sum up: Season 4 Ends (and Titles etc are calculated) with the launch of the next content patch (3.0). This is some time before WotLK is due to launch. Between the launch of Patch 3.0 and November 13th Arena Ratings and Points will be accumulated as normal and can be spent as normal. However come Nov. 13th all teams and ratings will be wiped, and the next Arena Season (S5 I guess) will not start until lvl80. We can assume that Arena Points still won't be wiped until you reach lvl71.

I can't say I didn't expect it TBH, but it will be very interesting to see how competitive all classes are in this brief period (~2-4 weeks). Also we can take the announcement of the end date of S4 as the 2-week warning for 3.0.

EDIT: Hints of a new Matchmaking System for the Arena?

There will be a new, persistent rating system that should keep top-rated players facing other top-rated players, regardless of the bracket in which their current team sits. We'll have more information to come on this soon. (Source)