Simple bit of info, often hard to find:
ESX 4.0 = Build 164009 - Released 21 May 2009
ESX 4.0 Update 1 = Build 208167 - Released 19 Nov 2009
ESX 4.0 Update 2 = Build 261974 - Released 10 June 2010
ESX 4.0 Update 3 = Build 398348 - Released 5 May 2011
An easy way to find build numbers without logging into the VI Client is to run:
vmware -v
See here for VMware ESX 3.5 Update to Build Number.
- Chris