Timeline: WoW client builds information

World of Warcraft Final builds information

World of Warcraft - Vanilla (Alpha)
6005 Version 1.12.2 (6005) Oct 13 2006

The Burning Crusade
8606 Version 2.4.3 (8606) Jul 10 2008

Wrath of the Lich King
12340 Version 3.3.5 (12340) (Release) Jun 24 2010
(a.k.a. 3.3.5a)

Cataclysm
15354 Version 4.3.3 (15354) Feb 22 2012
???? Version 4.4.0 (????) TBA

Mist of Pandaria
15464 Version 5.0.1 (15464) Mar 17 2012
15508 Version 5.0.1 (15508) Mar 26 2012
15777 Version 5.0.1 (15777) Apr 1 2012
(don't ask me how, but i first got the 1. April update and then the 15554 update)
15554 Version 5.0.1 (15554) Mar 30 2012
15589 Version 5.0.1 (15589) Apr 6 2012
15640 Version 5.0.1 (15640) Apr 22 2012